Added support to POST request containing a body formatted as JSON

Credit: Thanks to @hellogerard for this!

This enable limonade to receive a POST request with body containing JSON data. The JSON string will be parsed into an array, which will become the $_POST in your app. You'll be able to retrieve and browser your json key as array key, and getting your values.

Note:  I've added this because I have a lot of fields with their value to send through a web service, and passing this into the URL would break my REST API. Now I'm able to have a clean web service :)