Add varnish stats module

Process the output of varnishstat, attempt to match all of the metrics
and report them. If possible report the per second averages.

Take the names straight from what varnish reports.

Only tested on varnish 3.0.2 on Linux.
Skip VBE metrics.
Update metric type for varnish counters
Handle VBE metrics
Clean up comment
Merge pull request #1 from gsandie/varnish_check

Add the varnish check