Mongodb authentication

Add a new option to connect to mongodb with username and password. If the properties are available init the `MongoClient` with a `MongoCredential` object (both username and password need to be present).
`ninja.mongodb.user=<USERNAME>`
`ninja.mongodb.pass=<PASSWORD>`

During construction of the `MongoDB` object provide the possibility to initialize morphia.
`ninja.morphia.init=true|false`

Minor spelling fix in README.md