replaced icon

I was in the process of making the Vim logo as a vector and I decided to make a Vico style one. 

![vico](https://f.cloud.github.com/assets/1425304/2324736/a86d6a34-a3cc-11e3-80e6-ad79507ef34a.png)

I sampled the colours from the old icon. So things are still that same brown shade.

I realize this is a big change, so I understand any disinterest in making such a change.