Improve settings parsing: allows and emphasizes MongoDB URI usage in configuration

Currently, it is difficult to configure the connection settings for Flask-MongoEngine. While a MongoDB URI is technically supported, it requires that a user specify a database name in addition to the one in the URI. 

This PR fixes this issue by both
1. Extracting the database name from the URI (as per Mongoengine)
2. Using a default database name of 'test', as you would expect. 

Note, I have also submitted pull request https://github.com/MongoEngine/mongoengine/pull/737 fixing #2 in the driver layer, whereI believe it should be handled.

This change has a number of consequences:
1. Reduced repetition in parsing the settings dictionary via an internal _create_connection function
2. The documentation is now clearer on how to configure Flask-Mongoengine, and more obviously supports the most common configuration pattern today (using a URI). 
3. The extension will work out of the box with no configuration at all, and simply do what you would expect! (Connect to the test database on localhost).