JRuby support.

Uses `MultiJson` in place of `YAJL`, which in the end is using `YAJL` for MRI and `json_pure` for JRuby.
Upgrades version number to 0.3.3.
Better gemspec readability.
Formatting fixed.
multi_json update
Merge pull request #3 from pmaciolek/jruby

JRuby support.