Should add an s3_uploads_stop to the mix

[designstuhl]

I was looking for a way to cancel the upload before it was completed and ended up having to overwrite the add: callback inside of the setUploadForm function. During my research I found that this gem is based off of the blueimp fileupload plugin which supports a stop option. It appears that this is what the s3_uploads_start is tied into ( blueimp fileupload start ) so it would only make sense to extend the functionality to include s3_uploads_stop and utilize the blueimp fileupload stop option.

Great gem by the way, works very well!