Add in raindrops

Raindrops is a real-time stats toolkit to show statistics for Rack HTTP servers. It'll give us some visibility into the unicorn queue.

To view it in dev mode, rather than 'bowl rummager', do:

bundle exec unicorn -l 3009

Then you can view the stats at:

http://search.dev.gov.uk/_raindrops
Remove the twiddle, that's not how we roll.
Improve clarity of comment.
Merge pull request #122 from alphagov/raindrops

Add in raindrops