Renamed Worker.fork_and_perform_job to Worker.execute_job.

As per your suggestion in [this PR](https://github.com/nvie/rq/pull/309#discussion_r9568431), I renamed ``fork_and_perform_job`` to ``execute_job``.

Does ``run_job`` sound better than ``execute_job`` though?

Will separate the dequeue and execute logic in a later PR when I have the time.