Issue #1: Add JSON object with IANA media types list

 * Added initial IANA JSON data (data/iana_cached.json). This is meant for offline use.
 * Added support for rake.
 * Updated README to include rake tasks.
Merge pull request #4 from TheTribe/feature/issue-1

Issue #1: Add JSON object with IANA media types list
Issue #2 - Add media type code generator for C#