Add graceful_term option so TERM works like QUIT

If Worker#graceful_term is enabled, SIGTERM waits for a job to complete
before exiting (just like SIGQUIT).

Worker#graceful_term defaults to ENV['GRACEFUL_TERM'].

This makes it possible to use resque with runit/daemontools without
resorting to a wrapper script.
Properly set graceful_term option
Merge branch 'master' into graceful_term

Resolved conflict with trapping TERM signals

Conflicts:
	lib/resque/worker.rb
Add legacy worker tests for graceful_term option
Merge pull request #1006 from ktheory/graceful_term

Graceful term (for master)