Integrate Raven-JS for error logging to sentry bug 942912

- Depends on ravenjs
- Sends a msg to Zamboni to proxy error messages to sentry through the VPN
- Binds to window.onerror but allows for custom logging using Raven.captureMessage(msg)
Merge pull request #387 from JaredKerim-Mozilla/942912

Integrate Raven-JS for error logging to sentry