Blender Git Loki

modify the python context access so invalid names will raise an exception rather then returning None.
this way the UI scripts are less likely to fail silently and wont let typos work ok.

also allow subclassing of the context, added a copy function,
bpy.context.copy(), returns the context as a python dict to be modified and used in python.

This also showed up an invalid brush member in the screen context.