Change the GitGutter color to make it clearer

I found hard to see the changes I did in the GitGutter cause the opacity and the colors, so I did some changes to it.
## Before:
![before](https://f.cloud.github.com/assets/1068973/2461984/81c36440-af7c-11e3-959e-c077464245c9.png)
## After:
![after](https://f.cloud.github.com/assets/1068973/2461985/841127a0-af7c-11e3-8a65-1ea20089c6e9.png)