Ability to add a hook to github

It automates lint setup by adding a webhook on user's behalf.

For now the hook url is hardcoded as "lint.herokuapp.com/git_statuses".

The usecase is not yet used anywhere. It should be called
when user clicks "Follow" on the repository list page.
Merge pull request #45 from X8/add_github_hook

Ability to add a hook to github