Added pause() method for ResumableFile

Last time I was trying to compute MD5 checksum with SparkMD5, as I mentioned here (https://github.com/23/resumable.js/issues/135#issuecomment-31875165) on "fileAdded". It worked for one file, and faild for few.
I needed a way to stop file from being uploaded after added to queue, until script finishes checksum computation...
So I added .pause() method. It doesn't remove file from queue, just makes it invisible for `$.uploadNextChunk()` method.