Update app and document icon for Yosemite

This is the icon I proposed in #41. I have created a new .icns file with slightly tweaked versions at 512@2x, 256@2x, and 128@2x. Smaller versions will scale down using the 128@2x version.

This is the 128@2x version:

![icon_128x128 2x](https://cloud.githubusercontent.com/assets/6104/4024361/5b3cccce-2bc5-11e4-8bee-d97869ead978.png)

I also updated the document icon.

Before:

![macvim-generic-old](https://cloud.githubusercontent.com/assets/6104/4024364/750391e2-2bc5-11e4-9900-f6bb12063de1.png)

After:

![icon_512x512 2x](https://cloud.githubusercontent.com/assets/6104/4024365/8b48731e-2bc5-11e4-9107-d78d019e8e0a.png)