Added parsing of nonlocal declarations
Added semantic analysis of nonlocal declarations
Extended semantic analysis for nonlocal declarations
Fixed type errors.
Added nonlocal as a python3 specific keyword. Added print as a python 2 specific keyword.
Capitalized error messages
Added type check tests for nonlocal and global statements
Merge branch 'master' into nonlocal2

Conflicts:
	mypy/semanal.py
	mypy/treetransform.py
Merge pull request #504 from spkersten/nonlocal2

Support for nonlocal declaration