Added ability to pass custom headers in request. Fixes issue #47.

I love this uploader and had a real need to be able to use custom headers (specifically the Authorization header) similar to another request you had received here: https://github.com/LPology/Simple-Ajax-Uploader/issues/47. I went ahead and modified my copy and realized it was fairly simple to address this, so am submitting a pull request with the changes. I added documentation to the README.md file and there were a few incidental spacing changes in SimpleAjaxUploader.js. As I am not sure what you use for minification, I figured it best to let you uglify it as you see fit.