This patch changes the "Head" of a DataStore to be a single Commit,
as opposed to a SetOfCommit. This has several consequences:
1) Commit() will only accept Commits that are descendants of the
current Head.
2) Calls to Commit() can now fail, so the method now has an additional
'ok' return value that callers must check. Whether ok is true or
false, the DataStore struct returned is the right one to use for
subsequent calls to Commit() -- retries or otherwise.
3) This rolls up the stack, so Dataset.Commit() can now fail as well,
and similar logic applies.
4) sync.SetNewHeads() also behaves similarly, since it can also now fail.
5) Examples now die on Commit() failures.
Also, removes the /dataset endpoint from server. It's deprecated, and this
patch would have required updating it, so instead just delete it.
Towards issue #147
Chris Masone