Upgrade knockout to 3.1.0
Properly terminate web workers

Now that `Chrome 34.0.1847.131 m` is out and fixes [361782](https://code.google.com/p/chromium/issues/detail?id=361792), we can put back the code removed in #1604 and properly clean up our workers.
Merge pull request #1645 from AnalyticalGraphicsInc/terminate-workers

Properly terminate web workers
Update CHANGES.md
Merge pull request #1643 from AnalyticalGraphicsInc/upgrade-knockout

Upgrade knockout to 3.1.0
Merge remote-tracking branch 'origin/master' into upgrade-topojson

Conflicts:
	CHANGES.md