Add heroku to Gemfile

For rake dev:db.

The gem is deprecated, but this means users don't have to install the Heroku toolkit separately.
Extract spec helpers

Soon to be reused in the comments spec.
Basic comment 'resolved' state

Intended to be used to mark a comment as addressed: either code has been changed or it's a wontfix.

Not visually pretty yet, and some code deduplication would be nice. Coming up.

Refs #26.