Make general semaphore (add an option to limit to # of resources)

[bradvogel]

Right now this module implements a binary semaphore pattern: only one resource is allowed inside the critical code block at a time. It would be nice to have an option that allows us to specify the number of resources that are allowed in, so a true semaphore. This API could be something like:

var testLock = new Lock({
  redis: 'redis://localhost:6381',
  namespace: 'mylock',
  number: 5
});

testLock.acquireLock(id, 60 * 1000 /* Lock expires after 60sec if not released */ , function(err, lock) {
   // Up to 5 resources are allowed inside the code block at time
});