Use WP 3.5 Media Library

The only downside of this is that the plug-in now requires 3.5+ (but who doesn't update, right? :) ). In any case its possible to provide backwards compatible by registering a script to be used pre-3.5 to use the old media uploader.

To summarise the changes:
 - Complete rewrite of admin.js and dev-version to use the WP 3.5 media library
 - Mark-up changes
  - Adds a hidden 'ID' field (this isn't really used, its just there to ensure the attachment is pre-selected when the modal is re-opened).
  - Removes the select box
  - Changes IDs of the field to `wp-audio-{id/url}`.
  - Updates admin.css
  - Removes `delete_post_meta()` as `update_post_meta()` doesn't add a row unless the meta data isn't already present. 
  - Removes the functions `get_mp3_files()` and `has_mp3_files()` - these aren't used anywhere now


I've tried to add the 'insert by url' tab in the media library but I've been able to figure it out. However, you are still able to just enter in a custom

Apologies for bundling the last two in - they're in separate commits so you can ignore them if you don't want those changes made.

I've not tested in this in IE. There shouldn't be any issues, but I'm always accidentally putting commas in places they're not needed and IE shouts at me.