Ability to use a 'mongo.uri' config property to configure ReactiveMongo

This is mainly aimed at Heroku usage as all (atm both MongoHQ and MongoLab) the Heroku plugins add a 'URI style' aka 'mongodb://username:password@host:port/db' config property for your app to use.

Obs. 1 - I have added the slf4j-api 1.7.2 dependency, otherwise it wasn't compiling for me. You can roll that back

Obs. 2 - I've seen that the ReactiveMongo MongoConnection allows a different username/password for each MongoDB host/instance. That won't work with the above URI format. All MongoDB instances must have the same username/password.