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 pull request #10 from nimnull/master

Added contrib module for extra data types
Merge pull request #11 from nimnull/master

Fixed None value error handling
Merge branch 'master' of github.com:Deepwalker/trafaret
Add test
Merge pull request #13 from Marx86/master

Default value can be callable
missing trafarets added to __all__
Forward trafaret: check for not set trafaret added;
Key trafaret fixes: default value may be None; several more tests added;
Update __init__.py

Add min_length and max_length parameters for class String to allow string length validation like in trafaret.List class. Sometimes it is useful, example: check password length.
Merge pull request #14 from popravich/several_fixes

Forward & Key trafaret fixes + tests
Update __init__.py

Checking for simultaneous usage of min_length and allow_blank has been added (as per our conversation with user popravich).
Added tests for new functionality for class String(min_length and max_length options). Added min_length and max_length attributes to the its sub classes Email and URL (otherwise they will fail within check_and_return method).