General tweaks

Now the render output has to be specified at project level. This will
be inherited by the managers by default (in the future managers will be
able to override this and specify their own output path). Minor edits
to the server and managers Task and Project models.

It is a more standard command.

Dev

Merge of respawn functionality

Added a new component, the manager, as agreed in a previous design session. This core functionality allows to a server to communicate with multiple clusters of workers.

Fixes #48.

Communication used to happen between dashboard and server, which
violated the design. Now every request for info about a worker goes via
the server, so it can be more efficiently cached.