add pagination to worker counts, preserves backwards compatibility

[wr0ngway]

With any significant number of workers (we have 1300, soon to be 8K+), workers.counts becomes unwieldy, especially in the webui.

I tried to preserve backwards compatibility, so workers.counts without any args returns the entire set like it did before.

I also tried to match existing signatures that have pagination (e.g. offset, count as arguments).

Depends on seomoz/qless-core#42

[wr0ngway]

If backwards compatibility is not an issue, the count should default to 25 like the rest of the paginated methods. When both offset and count are 0, it gets the entire data set

[wr0ngway]

Note that there is single test failing (subscriber_spec#'can be stopped'), but it is unrelated to my changes - it fails without my changes after updating qless-core to head. I don't know enough to easily figure out why