Comment on PR when config is invalid

* Hound users currently receive no feedback when their Ruby, JavaScript,
  and CoffeeScript configuration files are invalid. This commit
  introduces a new feature to address this disconnect:
  When a config file is empty or can't be parsed, Hound will leave
  a comment on your pull request indicating that the config file is
  invalid.
* Validate repo config in `BuildRunner#run`
* Add `RepoConfig#validate` and `RepoConfig#errors`
* Add `GithubApi#add_issue_comment` and `PullRequest#add_comment`
* https://trello.com/c/JLt3FVUb/397-comment-on-the-pr-that-breaks-configuration
I'm not sure that we need to comment if the config is deliberately empty. What do you think about only recording errors if parsing fails?