Fix proxy error condition and simplify method

Add new minion proxy test case.
Address remaining comments from #756

Rename Encoding to Codec, Versioning to ResourceVersioner.  Add
GoDoc.  Add Delete(key, recursive) to EtcdHelper
Merge pull request #808 from smarterclayton/minion_proxy_test

Fix proxy error condition and simplify method
Merge pull request #806 from smarterclayton/follow_up_to_encoding_abstraction

Address remaining comments from #756
Decouple apiserver from codec implementation

The apiserver on initialization must be provided with a codec
for encoding and decoding all handled objects including api.Status
and api.ServerOp.  In addition, RESTStorage objects may now
optionally implement Extract() or instead rely on the codec to
decode for them.