Support for FormData submit in XHR requests

Added "processData" flag to ajaxSettings object which if set to false, will disable any $.param() processing on XHR data object.

Allow the user to set the XHR contentType value to false enabling the browser to automatically determine its value.

setting contentType to false and processData to false will enable the user to submit FormData objects via XHR requests which is helpful when submitting large content data or file inputs via XHR requests.
Merge pull request #588 from mourjan/patch-2

Support for FormData submit in XHR requests