feat(scheduling): simple method of running a unit file

* Poll the /coreos.com/coreinit/schedule/ namespace in etcd for systemd unit files
* When a new unit file appears, write it out to the local filesystem and
  remove the key from etcd
refactor(agent): remove agent code from registry.go

* Create a new agent.go file that contains all of the agent-like behavior
* agent.go is still in the registry package
refactor(registry): Move etcd-access to Registry

Serialization of data into etcd now lives in Registry
chore(imports): alphabetize import order
chore(logging): Use the builtin log package

Several log lines had to change, as they were simply logging
duplicate timestamps.
refactor(registry): Move Address code to Machine

The logic for inspecting and parsing a system's interfaces into Addr objects
now lives in Machine.GetAddresses
chore(agent): Add some simple documentation of Agent