Multi file support

This adds support for multiple files stored in local storage. Files can also be deleted out of local storage.

Saving is _not_ automatically triggered by the Autosave feature.

The pull request also includes some style updates for the file-chooser modal, which applies to github, dropbox and googledrive integrations. I've attached an image of the github file chooser.
![Github file chooser](http://puu.sh/7dS4g/9e06d8527c.png)

And here is the file selector for local files
![Local file chooser](http://puu.sh/7dSsV/fe8219106a.png)

The "delete" text is a hover effect of the X button.

You can test this functionality by visiting my [Dillinger deployment on heroku](http://subdillinger.herokuapp.com).