Tests for deleting users and groups

@bencorlett, I know you said you'd write the tests for the pull request I submitted (Issue #65) - but I decided to give it a try myself.  It passes the unit test, but what I'm concerned with is how to test "return parent::delete()".  Since the delete method in Sentry is returning what it gets from parent::delete() I figured there should be an assert.  But the problem is during testing it seems that it will always return null - to me it just doesn't seem logical to always assert that it's null.  Let me know what you think.  Thanks.