Adds multi_json gem as a dependency and loads it automatically
* Loads the "multi_json" gem automatically so that the user need not
* bother

* Loads the helpers automatically in modular-style apps. These are
* loaded automatically in classic style code (check
* lib/sinatra/contrib.rb) and this change preserves consistency of
* style.

* Adds multi_json gem as dependency. Some tests fails since "yajl-ruby"
* is used. This is added as a development dependency.
Modified json_spec as per changes made in the Sinatra::JSON module
merge #96