Changes needed before 3.0 release

[iainbeeston]

• [x] Changelog
• [x] Remove support for yajl and multijson
• [ ] $ref autoloading is disabled by default
• [ ] Replace UUID module with something faster
• [x] Remove ruby 1.8 support
• [ ] i18n support for error messages
• [ ] Stop stringifying input hashes
• [ ] Stop autodetecting/autoloading from input string urls (extract to an extension?)
• [ ] Stop inserting defaults (extract to an extension?)
• [ ] Remove list-mode (extract to an extension?)
• [ ] Reduce the number of class variables in Validator
• [ ] Remove :list option