Changes to use UTC date/times in iso8601.

I apologize for opening yet another pull request but I thoroughly mucked up the commit history on the last one. I've rebased onto the upstream 2.1 branch to make things much cleaner. I've also got a branch going that uses the [TZInfo](http://tzinfo.rubyforge.org/) library to handle daylight savings time. You can compare it to the current upstream 2.1 branch here: https://github.com/chrisperelstein/octopress/compare/imathis:2.1...chrisperelstein:2.1-utc-dst. I'm not sure if I integrated it into the Jekyll plugin properly, but I think using TZInfo is the more flexible way to go.