Added new Fields and support for several JSON Schema keywords.

Support for custom validators.
See json-document fork: https://github.com/cahersan/json-document (pending pull request)

Support for new JSON schema keywords.
- enum 
- minimum
- maximum
- minLength
- maxLength

Added new fields:
- JSONURLField
- JSONSlugField
- JSONTimeField
- JSONDateField
- JSONEmailField