Add custom DeferredQueue

https://github.com/praekelt/go-api-toolkit/pull/24 requires different operation to twisted's implementation of DeferredQueue. This implementation will return a Deferred for put, which will fire when there is place in the queue.
No, rather put those in the tests that need them. I don't want to have to bounce between setUp() and the methods I'm reading to see what the values are.