alpha approach to scheduling services through coreinit

This is not actually intended to be used yet, so no documentation has been written yet. The internal API needs to be solidified first. 

There's only one global etcd namespace to dump service files right now (/coreos.com/coreinit/schedule/<service name>). This should be scoped to machine IDs before anyone starts to use it.

The biggets refactoring piece here is the splitting-out of the Agent struct from Registry. The Agent is the interface between etcd and other local services. It's access to etcd is still abstracted through Registry.

I've also started using the log package.