add pagination to worker counts, preserves backwards compatibility

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 https://github.com/seomoz/qless-core/pull/42