Re-enable zk:// urls for Mesos SparkContexts

This was broken in PR #71 when we explicitly disallow anything that
didn't fit a mesos:// url.

Although it is not really clear that a zk:// url should match Mesos,
it is what the docs say and it is necessary for backwards compatibility.
Add unit test for SparkContext scheduler creation

Since YARN and Mesos are not necessarily available in the system,
they are allowed to pass as long as the YARN/Mesos code paths are
exercised.