[JRO] PostDecorator#user_link now available

https://github.com/jayroh/thredded/issues/19

While in a view displaying a post you might want to link to a user, so
this method in the PostDecorator is now available to accomplish that.

* Clean some specs up.
  * <80 chars
  * create_lists dropped to 2 from 3
  * Lock the timezone for Chronic to UTC so full rake passes