Make converted registration more clever.

The converter decorator now registers the converter.  This makes it
possible to have two modules extending the converters at the same
time, because there's now need to pass the converters in kwargs
anymore.  Which means that there's no need to wrap the functions
anymore.
Merge branch 'master' of https://github.com/vmj/protobuf_extra

Conflicts:
	protobuf_extra/__init__.py