Add license

https://github.com/AmpersandJS/ampersand/issues/33

NOTE this is not the same license as the others, it's the backbone license, probably needs discussion as backbonejs annotated source has:

```
Copyright (c) 2010-2014 Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
```

Whereas the backbone.js file itself has what's in this PR