resque-lock integration

Hi Aaron,

Let me know what you think of this. My company had a need to have status jobs, but not queue them if they already existed.

The changes were a bit more involved than I had hoped, but perhaps you'll see a better way.

My changes also now require resque >= 1.18 due to needing the after_enqueue hook. My earlier commit message was mistaken about it being 1.17.1.

Thanks,
Trevor