Set program octicons to black

Turns

![screen shot 2014-05-15 at 10 38 38 am](https://cloud.githubusercontent.com/assets/64050/2988259/cb2c536e-dc57-11e3-9696-8416a4f5821b.png)

into

![screen shot 2014-05-15 at 10 38 45 am](https://cloud.githubusercontent.com/assets/64050/2988258/cb2b8ec0-dc57-11e3-80b9-51ee5e5dbe6b.png)


I don't like the blue because it's the same color as anchors throughout the site, and the visual cue is false--these are not links. 

/cc @fabianperez @tobiasahlin