Add PR import rake task

Jobs can be queued for all PRs in a given repo using:

  rake import:prs REPO=puppetlabs/puppet

Individual PRs can be queued by using PR=123 in addition.  This also changes
the PR data extract to read the JSON layout used by GitHub for listing PRs, as
well as the normal hook JSON layout.
Merge branch 'domcleal-import'

* domcleal-import:
  Add PR import rake task

closes #26
(maint) Update Gemfile to use https://rubygems.org
Update the bundle dependencies
remove redundancy in README
:fire: old logic
explicit is better
added fixture for comments
add spec and implementation for comment model
initialize a pull request when initializing a issue
delegate method to retrieve a comments pull request
these are already defined on the super class
delegate repo_name method to issue from comment
consistency is key
add author information to comment model
add predicate method to check if comment was on pull request
require dependency for pull request model in issue model
add trello comment job and spec
add controller to handle newly created comments
trello doesnt support markdown in their comment bodies