WIP: Query cache

### Adding Query Cache to Mongoid ! 
![pd6ivbb](https://f.cloud.github.com/assets/833383/1598530/3ccbd2a4-5334-11e3-8754-87e0e662d7c0.gif)

### Peding tasks
 - Add more tests
 - Add option to disable cache
 - Add new Log instrumentation for query cache