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.
Add spaces between tests
Merge pull request #217 from aarondav/mesos-urls

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.

Additionally added a unit test for the creation of all types of TaskSchedulers. 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.