sync
more meaningfull errors
Added StrBool trafaret
more descriptive error messages
Updated README with KeysSubset example fixed and description about  feature
Fixes
Fixed allow_extra behaviour
Increase version. Update changelog.
added tests for DictKeys
Merge branch 'master' of github.com:marcinkuzminski/trafaret

Conflicts:
	CHANGES.rst
	setup.py
	trafaret/__init__.py