Refactor 'browser guesser'

This pull extracts the GitHub specific guess code from GitSCM.java into a static class in the GithubWeb class. A thin adaptor class is added as RepositroyBrowserGuesser.java, which currently checks the supported GitRepositoryBrowser classes for match and returns the RepositoryBrowser that matches. As a trial, a Stash guesser is also implemented.
Why?