Fix vpath build of python stubs
Merge pull request #46 from till-varoquaux/master

Fix vpath build of python stubs
fix for python 2.6
Remove layer_none from the environment. Pop should only be allowed to shrink the stack.
pub3: Report when access locals accross file boundaries.
Got rid of pub's `bind`, `unbind`  and scoped `lookup`
Flattened the libs lookup table in the environment...