Make Nodes be a slice of pointers so that we can implement a fake versio...

Make Nodes be a slice of pointers so that [Flynn](https://flynn.io/) can implement a fake version of etcd for tests that use Node internally as the type for the tree of data stored: https://github.com/flynn/strowger/blob/a59db1f76f71a7439f71ae3aec552888f074e34f/setup_test.go#L31

Flynn has currently forked go-etcd with its own version, but I'd like to get them using the upstream again if possible. See here https://github.com/flynn/flynn-host/issues/14