Basic Mixcloud Integration

This branch completes a basic (but usable) implementation of Mixcloud integration, with:
- OAuth authentication
- Automatic uploading of recorded shows
- Manual uploading

There's a few spots that leave a lot to be desired:
- There's no error handling if an upload fails.
- Airtime is missing most of the metadata fields that Mixcloud wants: Description, picture, tags
- A Mixcloud account should perhaps be associated with a show instead of a global system-wide account (this idea comes from the Mixcloud team.)
- In general, the code has a lot of copy paste from the SoundCloud code, which already has a lot of nastiness in it:
    - LibraryController.php has all the context menu items hardcoded in it. 
    - utils/mixcloud-uploader.php has a terrible copy/paste of conf.php at the top to import our config when running on the CLI.
    - All the progress indicators are hardcoded for SoundCloud stuff, so we're just ignoring that for now.

Ultimately, the SoundCloud/Mixcloud features need to be refactored into a common base including some sort of general upload manager, but for now, this will have to do. (We're out of time...)