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.
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).
Merge pull request #15 from mysteri0n/patch-1

Min and max length for String