Add format keyword support

[OptimumCode]

Add support for format keyword annotations and assertions as described in JSON schema spec.

I have decided to change the approach: supporting all the formats defined in the spec is a huge amount of work. Because of that this task will be split into several small sub-tasks:

• Provide API
  • Provide an interface to implement a format validator and API to register custom format validators
  • Provide API for switching between assertion and annotation mode for format keyword
• Implement formats:
  • date
  • time
  • date-time
  • duration
  • email
  • hostname
  • idn-email
  • idn-hostname
  • ipv4
  • ipv6
  • iri
  • iri-reference
  • json-pointer
  • relative-json-pointer
  • regex
  • uri
  • uri-reference
  • uri-template
  • uuid