Handle trailing slashes and whitespace in GitHub hooks, warn on failures

This pull request builds upon https://github.com/jenkinsci/github-plugin/pull/24.
In job configuration, Jenkins automatically appends a trailing slash to the repository URL in the field labeled 'GitHub project'. This causes a matching failure in GitHubWebHook/GitHubRepositoryName.

This patch further relaxes matchers, allowing trailing slashes and whitespace in URLs.