Allow maximum number of workers to be configured per job type.

[jeremywadsack]

Addresses #3.

We have instances where we would like to have the maximum number of workers depend on the type of Resque Job. For example, we have remote instances that are spun up by a worker, do some work, and then terminate. We could scale this as much as we wanted. In the same project we have workers that are pulling data into the database and we want to limit the number of those so that we aren't overburdening the database server.

This modifies the Job module to include a max_workers method (that becomes a class method for the resque job class that it's extended into). By default this returns the globally set maximum, but can be overridden to define a different value for each class.