Feature: Chrome APIs

[sgammon]

This changeset introduces initial support for chrome APIs, as requested in google/elemental2#119. I'm filing it early to get feedback and make sure my approach is solid. I don't know if you guys have this on your roadmap or not. No worries if this PR is out of place temporally or the approach is entirely wrong.

Externs added from Closure Compiler:

• contrib/externs/chrome.js

Progress:

• [x] Patched extern file for chrome.js
• [x] Working build
• [ ] Investigate support for chrome_extensions.js

[sgammon]

@jDramaix, understood. Thank you for your response