Add support for master svn repositories

Pull request related to issue #8331.
I have added support into the exists? method (actually working_copy_exists?) for checking if the path is a master repository, using svnlook. It should be ok in most cases, since svnlook comes with svnadmin in nearly every distribution.
Style and comments
Merge pull request #83 from sodabrew/issue_8331_svnlook

Add support for master svn repositories