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 in this implementation. 

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.