Attention: big changes in __init__.py, almost no changes in import_pdb.py
and export_pdb.py

1. With respect to the propositions made by Campbell and Sebastian
(see http://projects.blender.org/tracker/?func=detail&group_id=153&aid=29226&atid=469)
I changed the code. The PDB IO has now the preset option in the file dialog
of the importer and exporter.

2. Furthermore, I have put all properties into one group, which is located
in bpy.contect.scene. 

3. Some code cleaning

Soon, I will put in a new option (next days): the user can decide whether the 
panel is always, once or never shown after a PDB file is loaded. Note that
the panel is essential for me and also my students. It helps quickly modifying
PDB structures.

Blendphys