Fix Issue #23 related to JsonFormatter returning text/javascript

* Fixes issue #23
* Can now add `JsonpMediaTypeFormatter` to the end of the formatters collection.
* Support for `application/javascript` and `application/json-p`
* Added more `TestCase`s for various combinations of JSON/JSON-P co-existence
* Updated README
* Fix build script warning and remove unused fields
* Bug: UriExtensionMapping for "jsonp" now fails