File Upload & Layer Support

This may not yet be ready for primetime but I'd like to get some feedback. Pretty simple update. [Dropzone.js](http://www.dropzonejs.com/) handles the uploads. The included styling makes for a decent looking interface too. I added a bit of drag styling myself since there wasn't any originally.

Dropzone has a while bunch of events that are triggered during file upload but I just picked out a few of the ones that seem most important for the time being and integrated them into `app.vent`. See the [list of available events here](http://www.dropzonejs.com/#toc_8).

After being uploaded files are currently saved to `/mock_file/images/`. There isn't yet any sort of categorization or filing, and files with identical names will be overwritten without any sort of warning. Not important though, as that functionality can be incorporated later. Files are posted to `POST /upload`.