Adds multi_json gem as a dependency and loads it automatically

* Loads the "multi_json" gem automatically so that the user need not bother with that

* 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.