involve any drivers.

Plus a few styling enhancements.

[[Split portion of a mixed commit.]]

* You only have to do layout.prop if you want 1 property, no need for row then!
* Use col, row, sub as variable names, not colsub, rowsub, row2 etc please.
* Povray Addon: Still used a lot of splits, you need no split when you only have 1 column! 

- remove/comment unused variables
- remove unused imports
- fixed some bugs using incorrect variables

This is the last piece that should allow some pretty extensive custom rigging
in the metarig, if advanced users choose to do so.

This allows advanced users to do certain limited custom rigging work
in the metarig, and have it transfer to the generated rig.
Custom properties are also copied when using the copy_bone function
in utils.
TODO: transfer drivers from the metarig as well.

The layer names are then used in creating the custom rig layer UI.  This is
useful for users that do not want to--or do not have the knowledge to--edit
the generated python script by hand.  It is also handy even for more advanced
users when regerating the rig over and over (which over-writes the script
and any hand-made edits).

Also misc bug fixes in some of the rig types.

This allows IK arms and legs to still function properly even when they
start with an entirely straight chain.

This rig replaces the old spine rig, and has a super-set of the old spine's
features.  The main new features are:
1. A separate over-all control for translation.  Due to the pivot slide
   feature of the spine, it didn't really make sense to have one of the
   spine bones be the control for translation, so I broke that out
   into a separate control.  This control also acts as a root of the
   spine in general, including for scaling and rotation.  If you
   want to grab the entire spine as one unit, this is how to do it.
2. The spine can now have more than two control bones.  The rigger
   can specify an arbitrary number of the spine bones to be
   turned into controls upon rig generation.
   Controls that are not at the end points of the spine
   are optionally (via an animatable switch) auto-rotated by the
   the end point controls, so animators can ignore them when they
   do not require that level of control.

This is useful for bone chains that need their parent-child relationships
preserved exactly.  For example, bbone chains, and chains that users want to
use auto-ik on.
Also moved the 'copy' rig type to the basic collection.  This will break
some existing metarigs, but it is very easy to fix.  Just change the 'copy'
rig type on bones that use it to 'basic.copy'.

Matrix().Scale --> Matrix.Scale

the rig.

- Flipped the roll values on the default human metarig spine and neck.  This
  makes bending the rig forward have a positive rotation value, and bending
  back have a negative one.  More in line with user expectations.
- Improved get_pose_matrix_in_other_space() to account for bones with
  "inherit rotation" and/or "inherit scale" turned off.
- Added a text file giving credit to people who have contributed to Rigify
  financially or otherwise.

Moved operator registration for rig UI into the register function.  Not
really necessary, since the generated script doesn't act as an addon.  But
it keeps everything in one place.
Also added an unregister function for absolutely no reason.

Clean-up of the IK/FK snapping code.  Should be much more maintainable now.

Also changed rig id generation.  Rig id's now consist of a random
alphanumeric string 8 characters long, with the smallest 8 digits of seconds
since the epoc (in hex) at the time of rig generation appended on the end.
This results in a 16-character string that is ludicrously unlikely to
have any collisions between rigs.  36^8 * 16^8, with the 16^8 being very well
distributed over time.  Ah... paranoia.

- Added IK/FK snapping (both directions) for legs.
- Cleaned up another operator so that it works with undo.
- PEP8 cleanups. 

Also cleaned up how some of the operators work.  They weren't playing nice
with undo.

So far just IK->FK for biped.arm.

setup (not yet added to the default human metarig).