Add stats-ruby gem

So that we can push stats to statsd.
Track timings for MongoDB queries we make

Add some tracking around MongoDB query duration so that we can better
understand which queries are slower than others.

Created a top-level Rails::Application `statsd` object for the
application to share.
Merge pull request #67 from alphagov/add-statsd-timings-for-request-queries

Add StatsD timings for MongoDB queries