Add a javascript hook to our message framework

@dimagi/team-commcare-hq 
This is the javascript analog of messages in Django.

Use the function `alert_user` to make a message appear on the page. This accepts two args, message, and emphasis.  Emphasis corresponds to bootstrap styling, and can be "success", "error", "info", or "warning"

`alert_user("Awesome job!", "success");`

I'd like a little feedback before merging, if possible.  In particular, where should this *actually* live?  The bit where I add in the Django messages has to live in this file, but the rest could go elsewhere (although it IS small).  Should I be sticking some of this in a `$(...)`?

http://manage.dimagi.com/default.asp?149536
Oh, nice.  I'd much prefer to use plain object, the knockout docs all used classes, so that's what I did, although I guess it shouldn't surprise me that javascript lets you use either.

Also, the previous code I had DID work - messages were being displayed correctly, I wonder if firefox's js engine just figured out what I meant.  Anyways, certainly much better to write *actually* valid javascript.