Add a logo

This logo was created by Isaac Durazo (https://github.com/isaacdurazo) and Sean Mize (https://github.com/mize) based on https://github.com/voodootikigod/logo.js

Fixes #819

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/849

Closes #849, closes #819.