Use octicon for plus sign

Preview:

![githubextins-before](https://f.cloud.github.com/assets/42596/2160978/e0304aba-94c0-11e3-8e39-e63cd227d864.png "Before") ![githubextins-after](https://f.cloud.github.com/assets/42596/2160984/ed6d8936-94c0-11e3-83a4-6fa95dff1a09.png "After")

Doesn't look much different but at least it matches with other buttons.

If you prefer other icon &mdash; [beer](https://f.cloud.github.com/assets/42596/2161186/22ef4e12-94c3-11e3-9ad1-644990311c83.png), for example &mdash; see https://github.com/styleguide/css/7.0