Fix clipboard error while running Vim in tmux

Running Vim whilst in byobu/tmux causes the clipboard not to work, failing with:

```E353: Nothing in register *```

This has already been discussed [here](http://stackoverflow.com/questions/11404800/fix-vim-tmux-yank-paste-on-unnamed-register), the [accepted answer](http://stackoverflow.com/a/11421329/758165) seems to solve it.

There probably should go more testing into this, maybe there are other edge-cases where this might break, but it worked for me both in tmux and directly in the Terminal (running Mac OS X).

Best Regards,
-Pit