Add listing in ent client and add metrics

Ent client:
- add listing of all the blobs in a bucket with a given prefix
- move building a blob url to ent client

Metrics added:
- number of successful doozer backups after starting doozer-backup
- duration of the last doozer backup in seconds
- unix timestamp of the last backup in seconds

In order to get the last backup the newest blob from ent is listed. If there is still no backup, first successful backup is  created and then metrics are exposed. 

@soundcloud/iss @ohm  @juliusv 
Actually, I don't see a reason to use gorilla (heavy external dependency) if all we want to do is serve Prometheus metrics. Just use the native Go handlers. See the example at the very top of the page here: http://godoc.org/github.com/prometheus/client_golang/prometheus