check for single win

When you run vundle in vim with only one window opened, even if it is empty vundle still creates split:
![split](https://f.cloud.github.com/assets/1726487/905417/ecbf3b54-fc24-11e2-833b-1711101d6bad.png)

I made small check for that. Now if you have one empty window vundle creates single window, if you had anything opened it acts the same as before.