Concurrency limiting tool(s)

It would be nice to have an analog of [DeferredSemaphore](http://twistedmatrix.com/documents/current/api/twisted.internet.defer.DeferredSemaphore.html) and/or [DeferredLock](http://twistedmatrix.com/documents/current/api/twisted.internet.defer.DeferredLock.html).

This could then be used to implement #8.
