Relax URL matching for GitHub Webhook triggered builds.

Prior to this patch, the code would only match URLs that ended with '.git', which
is not required by GitHub. This patch allows projects to use URLs with or without
the '.git' suffix, and also allows for 'http' URLs. In addition GitHubWebHook did its own parsing of the repository URL and passed the components to GitHubRepositoryName, instead of letting it do its own parsing.

Addresses the URL matching parts of JENKINS-10405, but not the security/authorization
parts.