Adding "format" keyword support

Improvements

• added support for the "format" keyword of the specification (requested in #22 )
• support for custom (non-standard) formats - [https://github.com/everit-org/json-schema#format-validators](see the readme)
• upgrading to JSON-java version 20160212 (it fixes #25 )

Bugfixes

• schema properties occuring in an object containing a $ref reference now extend the referred schema (fixes #21 )
• fixed incorrect error messages for schema violations where the schema forbids additional properties and the subject under validation has multiple additional properties (reported in #28 )