Added hint method on Find object
hint method allow user to specify which index to use when quering a collection.

ie: myCollection.find("{name: 'John'}").hint("{$natural: 1}").as(Friend.class)
Add a new rule which start an mongoDB instance in memory
 (which is a good stuff for uning testing !)
 Update unit test on hint method
Update of a test to really show an use of hint method.
Remove in memory mongodb
When running tests, it use the same port than real mongodb so it cause some tests issues if a read mongodb is running.
Merge pull request #126 from dwursteisen/master

hint method on Find command