allow connectionString in mongodb config, which solves the db authentication

As mentioned in #41, it would be nice to support mongodb authentication.

If we allow connectionstring it's possible, and mongoskin supports too.

if you want to use authentication or prefer connectionstring, use the config like this:

    var countlyConfig = {};

    countlyConfig.mongodb = "mongodb://foo:bar@localhost:27017/countly?auto_reconnect";