introduce Comment model
create Comment instances from GitHub reponses
refactor comments_count
introduce request_comments
move request comparison to base class
Remove method_missing from provider base

Don't call client from base.
Different providers use different clients.
Different clients use a different syntax.
Rather be explicit when doing things like this.
simplify URL construction
improve test coverage for URL construction
refactor authentication
delegate PR creation
add bitucket skeleton
move url helper to provider base
add more tests for authorization