mime-types 2.0

A substantial upgrade to the mime-types library. Major features:

- No longer compatible with Ruby 1.8.
- Several major API changes and deprecations.
- The default mime-types registry is now stored as JSON.
- Improved developer tools.
- Improved repackager tools.
- Fixes #28.
- Fixes #36.
- Fixes #37.
- Fixes #43.
- Fixes #44.