First stab at writing implementing job dependency.
Don't enqueue waitlisted jobs on failed execution.
Avoid race conditions when enqueueing job with dependency.
Simplify enqueue_waitlist by using lpop.
Internally renamed the term "parent" to "dependency".
Merge branch 'master' of git://github.com/nvie/rq into job_dependency

Conflicts:
	rq/queue.py
Don't fail if job dependency is modified during enqueue process.
Merge branch 'master' into job_dependency

Conflicts:
	rq/job.py
	rq/queue.py
Python 3 fixes for job dependency stuff.
Merge pull request #207 from selwin/job_dependency

Job dependency