Adding MongoDB TTL index

I have same suggestion as https://github.com/hapijs/catbox-mongodb/pull/5

ensureIndex is called only once per process life-time. 

As described here http://docs.mongodb.org/manual/tutorial/expire-data/ , every document requires expire date (I created expireAt field), so individual items can have individual times. 

Its my first day with Hapi, so hope its ok. I will need some assistance with code coverage though. It makes sense to test err incase it drops connection or something, but not sure how to write test for it.



I familiarized myself more with existing tests, so if you confirm that changes I proposed looks good to you, I can spend some time on tests to fix 100% coverage.  

In this case its necessary to cause err coming from ensureIndex. By looking to existing tests, it seems that there is no more elegant way than just fake ensureIndex function to generate callback with err, or is there better trick? :)