Sending varnish logs to logstash

Formatted varnish log output to logstash JSON format. Also had to setup
a filter to convert varnish timestamp to iso8601. The version of varnish
we are using (3.2) doesn't support specifying a time format and 3.4
hasn't been released yet.

Lumberjack ships the logs to logstash.
Merge pull request #90 from alphagov/varnish-to-logstash

Sending varnish logs to logstash
(roc: One day we will have naming conventions)