Add support for ReadPreference - fixes #116

I've added ReadPreference support in MongoCollection as discussed. For example:

```
collection.withReadPreference(ReadPreference.primaryPreferred()).count()
collection.withReadPreference(ReadPreference.primaryPreferred()).find(...)
collection.withReadPreference(ReadPreference.primaryPreferred()).findOne(...)
```
Concerning the unit tests: I didn't find a way to check that the ReadPreference hint is really used by Mongo driver. So for the moment I just added tests to check that the API works as described above. Any ideas to improve this are welcome :)