mongoengine.MongoEngine.connect() missing positional arg

I'm using mongoengine 0.8.6 and flask-mongoengine 0.7 for this.

I found that the [documentation](http://docs.mongoengine.org/apireference.html#mongoengine.connect) requires a db name as a positional arg, and `flask_mongoengine.MongoEngine.init_app()` is not providing this positional argument, resulting in a TypeError.

This attempts to grab a DB name out of the settings dict, and defaults back to the app.name.