Bug 5216 - add instagram as comment-embed source

Added instagram to the whitelist and added a URL to the test suite.

![screen shot 2013-09-02 at 4 04 15 pm](https://f.cloud.github.com/assets/185758/1069604/5965c9a0-1424-11e3-8933-afc5c5e5e9ad.png)