Add post drag n drop support

* after examining several plugins that either don't have good direct upload to s3 support or doesn't have a drag-and-dropzone functionality without a form and input field I rolled my own. The only thing I'm using is jquery.fileupload which handles file uploads rather well but doesn't have out of the box s3 support.
* The upload is very much github style when the you drag an image it shows a placeholder for "uploading batman_flask.jpg" and then this is replaced with the actual markdown image reference when the upload is completed.