feat(alarms): add /entities/{entityId}/alarms endpoint

See the Cloud Monitoring docs, section [5.12.3](http://goo.gl/9TUYBP).

I want this for a new feature Cloud Intelligence is developing that needs to be able to get all the alarms for a given Entity.
I count 22 uses of the function, let's change it in a separate PR to avoid blowing up the diff.