Several changes

I've added several fixes/additions. Here's a complete list:

 * [Issue 34: show changes that were saved to disk](https://bitbucket.org/sjl/gundo.vim/issue/34/show-changes-that-were-saved-onto-disk)
 * [Issue 38: Python errors NoneType](https://bitbucket.org/sjl/gundo.vim/issue/38/python-errors-nonetype-not-iterable-with)
 * [Issue 40: live updates](https://bitbucket.org/sjl/gundo.vim/issue/40/feature-request-live-reload)
 * [Issue 28: open vimdiff of current buffer..](https://bitbucket.org/sjl/gundo.vim/issue/28/feature-request-open-vimdiff-of-current#comment-3129981)
 * [Issue 33: fix for global_disable](https://bitbucket.org/sjl/gundo.vim/issue/33/let-g-gundo_disable-0-is-not-available)
 * [Issue 31: reduce overall verbosity](https://bitbucket.org/sjl/gundo.vim/issue/31/reduce-verbosity-of-the-list)

Haven't heard anything from the first pull request - maybe I'd do better over on the bitbucket site...I'll cross post there...