Added small integration test for .every() jobs.

Let me know what you think about this test, I'm OK with refactoring it if you have a better idea of how to test this.  I ported it from my own application tests, ensuring future agenda updates don't cause unhappy behavior (we are expanding our use of .every() calls).

I also removed the `agenda-test.js` file from the repo as well, as it looks like it was the proof of concept script I had opened with #70.  This PR uses this same idea behind that.

If you want to check to ensure these tests cover the use-case, you can look at the `failing-test` branch on my fork of the repo.  It's based off the `0.6.14` tag which had a broken `.every()` implementation.

I can also start looking into covering the other methods such as `.once()` `.now()` and `schedule()` calls.