Stats & profiling.

Still seeing spider thread stop, still no reason given for why.

So, import net/http/pprof which gives us /debug/pprof/ URLs, with
  /debug/pprof/goroutine?debug=1
being roughly equivalent to the old Python version's /threadz.

We also get some other human-readable output at links from /debug/pprof/

Also, import expvar and set some basic stats counters from spidering.

SECURITY: definitely do not pass all of the URL space through from the
front-end, only /sks-stats as strongly suggested.

[ Affects issue #3 ]