Merge pull request #25 from zeliard91/master

Handle null and not_null comparison operators

Fixed #24: Throw exception when value cannot be decrypted.

Fixed #17: Add JSON renderers and transformers.

Fixed #23: Add a date transformer that converts DateTime objects and …

…textual datetime descriptions into Unix timestamps.

Fixed #20: Add a class-level cache for the normalized entity classes.

Updated readme.

Fixed array flattening bug.

Added the date renderer and password transformer.