[#2] Implement github authentication

OAuth secrets are obtained from environment variables
- AppID     -> ENV['GITHUB_ID']
- APpSecret -> ENV['GITHUB_SECRET']
[#7] Add Lint::PullRequest to recognize git sha

When we get message that there was commit in repository we can recognize
base and target SHA's to list of modified files.
[#7] Introduce Lint::Patches

With this class you can get files related to given pull requests.