Commands

Fixes #42 

I still need to add more Command+Consumer specific tests, but this has so many changes in it I'd prefer to merge this ASAP.
I moved it into the main watching goroutine: https://github.com/lytics/metafora/pull/46/files#diff-f06c1807509d1d15caeedbdc6515e861R42

The downside is that I have to awkwardly create fake etcd Responses to wrap the Nodes in.

The upside is that all etcd task-grabbing is done in a single goroutine.