Features/mongoengine

This is quite a few changes.

I've ported to using MongoEngine.

The bookmarklet on the front page does not appear by default anymore.

On login, you'll now get a dialog popup and the bookmarklet link will show up on the front page.

Users are all stored in mongo using mongoengine.

Duplicate logins are special cased - you just get the first login information.

Custom bookmarklet per user works.  

Added mongodb to the travis services so that tests run against a real mongodb instance now.

I haven't tested this on heroku.