Added if statement to check if Sublime symlink needs to be added

Hi Paul, thanks for a some really cool dotfiles!

This change will check if the sublime symlink exists before adding it (assuming it is not commented), so the message in the attached image never shows. What do you think about it?
![dotfiles-paul-symlink](https://cloud.githubusercontent.com/assets/1140124/5019055/4986b432-6abc-11e4-9cb1-f4379bc8859a.png)

Best Regards,
Niklas