Change gutter warnings/errors to color the line numbers

Changes the warnings/errors in the gutter so instead of additional dots being shown next to the line numbers, the line numbers themselves are colored. I think this has several advantages:

* Integrates more nicely with atom.io, for example the folding feature of atom uses something similar
* Makes the gutter smaller, leaving more room for content
* Less intrusive while working, still easily spottable when needed
* Removes font-awesome dependency

Example:

![screen shot 2014-06-12 at 11 13 01](https://cloud.githubusercontent.com/assets/229105/3255980/eea6009a-f211-11e3-8834-0782aa4fa51b.png)

PS: This is my first pull request, be patient if something is not as it should please ;-)