Use MultiJson to improve JSON performance when possible

This pull-request is a response to #99. It simply replaces OkJson w/ Intridea's [MultiJson](https://github.com/intridea/multi_json).

From the [README](https://github.com/intridea/multi_json#multijson--):
> MultiJSON tries to have intelligent defaulting. That is, if you have any of the supported engines already loaded, it will utilize them before attempting to load any. When loading, libraries are ordered by speed. First Oj, then Yajl, then the JSON gem, then JSON pure. If no other JSON library is available, MultiJSON falls back to OkJson, a simple, vendorable JSON parser.

As an example, to leverage the awesome [Oj](https://github.com/ohler55/oj), add `gem 'oj', '>= 1.3.5'` to your app's Gemfile and `bundle install`. MultiJson will automatically use Oj for QC's JSON processing.

@ryandotsmith I didn't touch the tests. Would you like me to add one to assert that MultiJson is working properly?