[WIP] Implement support for read preferencesUse separate Cursor property for ODM's UoW hints

This PR will require:

 * [ ] Merge doctrine/mongodb#105, which separates MongoCursor hints from the Cursor property used for ODM hints
 * [ ] Implement read preference support in doctrine/mongodb's query builder
 * [ ] Tag a stable 1.1.x release of doctrine/mongodb
 * [ ] Implement read preference support in ODM's query builder
 * [ ] Extend ODM hints to support read preferences (for consistency between queries/cursors and PersistentCollections)