Switch to Rack::Parser

I'm just curious why you implemented your own body parser instead of using [Rack:: PostBodyContentTypeParser](https://github.com/rack/rack-contrib/blob/master/lib/rack/contrib/post_body_content_type_parser.rb). The biggest difference I guess is being able to pass in your own json parser.

Edit: [Rack::Parser](https://github.com/achiu/rack-parser) uses MultiJSON and MultiXML and allows you to use any custom parser.