Replace go-etcd

This replaces the dependencies on github.com/coreos/go-etcd and github.com/coroes/etcd with a new local package surprisingly called "etcd". It is only intended to implement the functionality that fleet needs (for now).
@bmizerany The 0 at the end of my etcd.Set example compared to the 0 in the CompareAndSwap example demonstrates my point. I have no idea what that zero means without reading documentation. I'm not against creating more specific methods, but then we have to be careful not to create an interface with 5 times as many methods as are actually needed.