forms: Drag and drop uploads

This replaces the multi-file upload widget for a new-fangled drag and drop widget. The new widget supports a few super-powered features

  * Multi-file drag and drop
  * Multi-file select (for real this time)
  * Progress bar
  * Trash can for removal
  * Link for preview

The widget is not connected to the Redactor editor, so that the editor can be disabled and the upload will still function properly.

### Outstanding
  * [ ] Internet Explorer 9 support (FileReader javascript class is required). More help [here](https://developer.mozilla.org/en-US/docs/Web/API/FileReader)
  * [ ]  Integrate thumbnail and preview support for uploads
  * [ ] Upload cancel button
  * [ ] Fallback support for unsupported browsers (upload on submit)

##### Upload Progress
![screen shot 2014-08-21 at 12 56 15 pm](https://cloud.githubusercontent.com/assets/672074/4000762/12a1e4f0-295d-11e4-918a-1e4ec7cbb4b8.png)

##### File list view with removal option
![screen shot 2014-08-21 at 12 56 41 pm](https://cloud.githubusercontent.com/assets/672074/4000768/1c6f1534-295d-11e4-9308-536564459788.png)