Improved vim syntax file.

This fixes a few problems with the old vim syntax file, and tones the colours down a bit.

@matthewrmshin et.al. (anyone use vim?) - please review.

Terminal:
![vim-syntax-term](https://cloud.githubusercontent.com/assets/2362137/3915886/5f808c0a-236c-11e4-93b7-a481ed83e278.png)

gvim:
![vim-syntax-gui](https://cloud.githubusercontent.com/assets/2362137/3915887/67ec09f0-236c-11e4-816f-ea3fe2ba6a97.png)