Make httplog more useful by using glog.*Depth.

Close #2100 

With this change, the initial log line: 

I1213 00:05:02.919080    8245 log.go:151] GET /api/v1beta1/minions: (10.922746ms) 200

Convert to the following:

I1213 00:07:41.092850    9845 handlers.go:121] GET /api/v1beta1/minions: (15.214229ms) 200

cc/ @lavalamp