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
What I was thinking... we could add an error right here, when the parsing of config fails. Thoughts?

Were you thinking that if the file exists, but looks empty, then we'd want to notify the user about it?