Adding emoji support using Twitter's twemoji

This commit adds emoji support to GB using Twitter's [twemoji](https://github.com/twitter/twemoji) library.

Emoji characters are scaled to the font size via CSS.

Currently, this parses every incoming line for every supported emoji character. I can make it an option in the settings pane if there's a reason to not auto-emojify everything. Additionally, characters can be excluded if needed.

Example:
![image](https://cloud.githubusercontent.com/assets/176014/5059933/1c56b2e2-6cfa-11e4-8b34-b3473ddb9f38.png)