Adds an after fork hook for workers.

resolves conflicts of cherry-picking

Conflicts:

	lib/qless/worker/forking.rb
	spec/integration/workers/serial_spec.rb
removes redundant test
fixes worker/forking_spec

The forking worker spec, under the previously proposed changes couldn't actually
go red. This corrects the test, so it can actually go red withot the proposed
changes.

Also, this consolidates the forking code.
Merge pull request #149 from seomoz/after_fork_hook

Adds an after fork hook for workers.