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

[cillosis]

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: #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.

[LPology]

Thank you very much for this.

I just merged your PR, but there are a couple of changes I'm about to push. When looping through objects, it's good practice to use hasOwnProperty to distinguish between properties defined for the object and those inherited via the prototype:

    for ( var headerName in opts.customHeaders ) {
      if ( opts.customHeaders.hasOwnProperty( headerName ) ) {
        xhr.setRequestHeader( headerName, opts.customHeaders[headerName] );
      }
    }

Also, just in case you're curious, I've been using YUI compressor to minify. It seems to work well. Here's an online version:

http://yui.2clics.net/