Added from-python conversion of RunTimeTypedClass to IECore::TypeId.

This makes calls to things like RunTimeTyped.inheritsFrom() a little more Pythonic, without littering staticTypeId() calls all over the place. The main motivation for this is to replace the isolated code from here :

https://github.com/ImageEngine/gaffer/blob/master/src/GafferBindings/MetadataBinding.cpp#L114

This will allow us to make the Gaffer API more consistent - currently most registration functions implemented in Python take classes directly, but those implemented in C++ and bound to Python require TypeIds. Accepting classes everywhere will make the API simpler and more uniform.