DRY-out settings parsing and creation of mongoengine.connection
Extracts db from MONGODB_HOST if it is a mongodb URI
Creates a default database name for connections, named 'test'
Adds tests for default database connection settings. I.E. unconfigured
Adds documentation for connection methods, reusing mongoengine examples
Merge pull request #104 from wcdolphin/uri-handling

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