Add feature to respect dependency

[rontendo-jp]

rq-scheduler should respect the depends_on parameter like in RQ.

Job B which depends on Job A should not run if Job A has not finished.

DETAIL:
A simplified use case, there are two jobs: Job A, and Job B. Job B should not run before Job A is done.
In an environment where there are more than one rqworker running, without this code change, rq-scheduler doesn't respect job dependency. Meaning that, since there are multiple rqworkers running, Job B and Job A could run in parallel.

SIMPLE TEST:
I created like the below, and confirmed that job2 did not get picked up by the second rqworker if job1 has not finished.
job1 = s.schedule(scheduled_time=START_TIME, func=count, interval=SCRIPT_INTERVAL, repeat=1)
job2 = s.schedule(scheduled_time=START_TIME, func=say_hello, depends_on=job1)

If you like this PR, I can update the README.md afterwards. Thx!

[selwin]

Thanks for the PR. Could please add a test for this feature?