Add a command to visit the homepage of a Bundle

The command BundleVisitHomepage can will try to take the user to the homepage
or directory the bundle is cloned from.

This is not yet system independent and can not yet handle full git uris.

**questions and ideas:**
This is still work in progress. Do you think it is a feature that should be included into vundle?

 It might be interesting for people who demanded some information on the plugins inside the search window where some key could be mapped to this function.

If you have ideas how to handle general git uris or open uris on different systems please comment. (What can you do on windows to open an uri? On linux we might try for `xdg-open` or are there many window managers and desktop environments that don't support it?)