Updates to comment view design

* Username in header is now bold to match calypso design. 
* Moves timestamp to below username.
* Commenter's Url is displayed next to the timestamp, if available. When tapped it loads the Url in a web view.
* Removed gap between notification header and content to match new calypso design. Thanks @jleandroperez!
* Fixed incorrect `separatorInset` on iOS 8.

New look:
![screen shot 2014-11-05 at 9 11 46 am](https://cloud.githubusercontent.com/assets/789137/4922176/d8c89028-650f-11e4-8cba-03658286b496.png)