Video tracking tweak, GA experiment added. Bug 865433.

- Avoid pause tracking when video ends.

- GA experiment code added to base /firstrun page.
Merge pull request #868 from mozilla/bug-865433-firstrun-updates

Video tracking tweak, GA experiment added. Bug 865433.
Fix bug 859572: Redirect tabzilla.js to CDN.
Merge pull request #851 from pmclanahan/redirect-tabzilla-cdn-bug-859572

Fix bug 859572: Redirect tabzilla.js to CDN.