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 qless ruby client's 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).

However, as a result of the above two requirements (?) the QlessWorker.counts argument processing is a little hokey - basically I check wether the first arg is a number to determine wether the user intended to paginate all or get a single worker.