add config and gem to use Grape::Formatter::Rabl

add gem 'grape-rabl'
add some command in config.ru to load
to make rabl file index.json.rabl to reuse the rabl template fish.json.rabl
add the argument to specify use rabl tamplate in api.rb
add configuration for use grape-rabl in config/application.rb seeing https://github.com/LTe/grape-rabl
modify arguments in api.rb and api/index.json.rabl related with file extension while referring http://qiita.com/anoworl/items/756f01cc3d188ebad139