Add bundle exec rake jobs:summary task

Without this patch we don't have an easy and automated way to summarize
all of the cards completed in a given time.  This patch addresses the
problem by adding a `jobs:summary` task.  This task will scrape all of
the cards in the `TRELLO_FINISHED_LIST_ID` column and create a summary
of the results.  It will use a Liquid template from
https://github.com/puppetlabs/puppet-webhooks/blob/templates/templates/trello_template.md.liquid
by default.

The URL may be overriden using `SUMMARY_TEMPLATE_URL` with the rake
task.  e.g. `bundle exec rake jobs:summary
SUMMARY_TEMPLATE_URL=http://jeffmccune.com/summary.template`