use listen gem to monitor config file and dynamic adjust resque workers count

[comme]

in my project, i must keep resque queues don't have queueing job,so the workers count shoud greater or equal jobs count, i kone send -HUP to resque-pool-manager can adjust workers count but i think resque-pool self supoort dynamic adjust workers count is better!

[nevans]

I like it! I should take a closer look at it tomorrow, and I expect I'll merge it and roll it into the next release then. Thanks!

[nevans]

I'm finally getting the chance to look into this. Two minor concerns:

1. When the listen gem is running in non-blocking mode, it executes the callback in a different thread. Therefore the callback should be 100% threadsafe. Since all we're doing is calling the HUP code anyway, I'm going to fudge that by changing the callback to add a :HUP onto the sig_queue... and change the sig_queue from an Array into a thread-safe Queue.
2. People who don't use that command-line option shouldn't need to have the gem. I'll move the require inside the :dynamic_adjust_workers_count= setter, and I'll move the dynamic_adjust_workers_count? accessor forward in its if statement for short-circuit purposes.

[nevans]

Sorry, I didn't get around to making my stated changes last night. If you make those changes, I'll merge it in. Otherwise, I'll get around to it the next time i have a spare moment.

[nevans]

I do think this is obsoleted by #85. I hope to investigate that PR and merge it (or something very like it) into master soon. This code could then be relatively easily reimplemented more cleanly on top of that code (and I'd gladly include an example config in the docs, or a --listen command line flag to automatically enable).