Added small integration test for .every() jobs.

This test .fork()'s a new instance of Agenda that runs a
.every() job once day located in the /test/fixtures folder.

The forked process either runs the job and sends a "ran" or
"notRan" data event, notRan relying on a setTimeout() that gets
ran after 300ms.  After being ran or timing out, the child
process exits.

Tests could be further expanded to ensure .every() gets called
at the set interval as well.  There also could be integration
tests that check .once(), .schedule(), and .now() methods as
well.