Use MultiJSON for JSON parsing

By specifying a runtime dependency on `multi_json` instead of the `json` gem, it gives more flexibility to the end-user to use whatever JSON library they prefer (falling back to `json` on Ruby 1.9 and `okjson` on Ruby 1.8). In addition to these two adapters, `multi_json` supports Oj, YAJL, NSJSONSerialization (MacRuby only), and gson.rb (JRuby only). This allows for greater compatibility across interpreters, which may not support MRI's C extensions.

This pull request was created in response to #359.