Add MIT License.

Closes #3
[docs] Added better documentation for how to create an App ID

* Fixed an HTML bug in set-app-id.html
* Added short instructions to set-app-id.html for creating an app
* Added instructions to README.md
[docs] Corrected typo
[vendor/inboxjs] Update submodule to new version
Fixed linting errors

A clean lint gives me happy fuzzy feels
Updated submodule
[coffee] Converted baobab.controller.compose to coffeescript

Turns out the reason we had trouble with this before is that `this`
points to the controller during construction but to window during
execution. Gnarly.
[fix] Removed a debugging element that shouldn't have been committed
[feature] Improvements to drag-and-drop

* Disabled a default fallback message of dubious tone
* Restructured file list DOM to have the same structure as for attachments outside an editor
* Switched off image previews