Replacing mailru/FileAPI with native XHR2 upload.

This was made possible by completely removing IE8 support. The new
upload functionality has been tested against Mozilla Firefox, Google
Chrome and Internet Explorer 10.

Removing this additional dependency should also make the upload
behaviour somewhat easier to understand.

Incidental improvements include:
 * Failed files show red in the progress bar.
 * Progress bar now remains hidden until used.
 * Resolves #128.