Add /stats, /stats/queues, & /stats/queues/:name json endpoints to web

Fixes issue #2098 
I like the general idea here but it's pretty slow for people with lots of queues.  N queues means N round trips to Redis.

What do you think about keeping /stats for just the basic stats needed for the dashboard?  /stats/queues can do the hard work of gathering the queue sizes.  This way if the user just wants basic stats, they don't need to pay that overhead to gather queue data.