Add __all__ decl in libs.py.

When we have no __all__ decl in libs.py, the top level blaze module pulls in all
asunder which isn't what we want.
Add __all__ decl in blaze error and correct names.

InvalidLibraryDefinition was mispelled as InvalidLibraryDefinton.
Merge pull request #9 from ehiggs/devel

Add __all__ decl in libs.py, error.py