Return false if the fork function is unavailable

[chrisboulton]

Deals with #93 and #94

[lauripiisang]

Good stuff, but in bin/resque, shouldn't the program at least say something if you are using a forking OS, but don't have the appropriate module ? E.g. darwin or unix + no pcntl_fork extension.
With this patch, it turns from a fail into a silent fail, leaving people scratching their heads over performance.

[chrisboulton]

Honestly, I think anyone using bin/resque to manage more than one worker is asking for trouble and should really be using a real process manager to manage a pool of workers (monit, daemon tools, something else). I've gone ahead and pushed up a "fix" for the above so it at least bails out. That'll do for now.

[lauripiisang]

Honestly, I think anyone using bin/resque to manage more than one worker is asking for trouble and should really be using a real process manager to manage a pool of workers

Perhaps this should be documented in the README under "Running multiple workers".