This change makes the etcd package compatible with the existing Go
ecosystem for module versioning.
Used this tool to update package imports:
https://github.com/KSubedi/gomove
Add a bunch of tests on fundamental operation of Create/Set/Delete.
Context: I want to use this package to run the discovery service against
the v3 storage backend. The limited functionality already implemented
should be sufficient to do this.
https://github.com/coreos/discovery.etcd.io/issues/52
