Authenicated as SYSTEM user for github webhook.
Merge remote-tracking branch 'origin/master' into bugfix/authenticate_as_system_on_github_webhook

Conflicts:
	src/main/java/com/groupon/jenkins/github/GithubWebhook.java
Merge pull request #79 from groupon/bugfix/authenticate_as_system_on_github_webhook

Authenticate as SYSTEM user for github webhook.