More metrics

Changes:

1. Added gauges to track total enqueued, processed,
   and failed jobs, and 2 queue-level metrics - depth
   and latency. These metrics reflect numbers that
   Sidekiq::Stats api gives us.

2. Removed the Client as it's main responsibility
   was to return a statsd client which is done
   from the middleware. we don't need wrapper
   methods for increment, gauge, time as the
   Client had, because those were only adding
   correct prefix, which is replaced by a Util.

3. Because we have a bunch of metrics to track,
   it would be better to use the statsd batch mode
   which came with the 1.1.0 release of statsd-ruby.

4. Added a new option statsd, to enforce the usage
   of an existing statsd client.