feat(audio.coffee): report upload progress
feat(audio.coffee): report upload progress

This one was tough. There's no way to do so using `fs`, and there's no public File API method to
programmatically select files for file input.
Use the (non-standard File API) File constructor to represent an audio file, then use
XHR and FormData to upload it.

deltaidea/vk-sync,deltaidea/vk-sync