hint method on Find command

Hello, 

I just added the hint method on find command (to select which index to use)

```java
myCollection.find().hint("{nameOfFieldIndexed: 1}").as(MyObject.class);
````

Please note that the pull request contains a new junit rule which allow to start an in-memory mongodb. 

```java
@Rule
public static final MongoInMemoryRule mongo = new MongoInMemoryRule();
```

Due to the mechanism used in all test (ie: use the default mongodb port), it's in conflict with a true read mongodb instance. But I leave it in the code, because it can be usefull for other test.

Regards