Jekyll 2.0 compatability

For a new installation this appears to work fine. I still need to do some testing. Anyone who's willing to try out upgrading to this branch, please post any issues and I'll try to address them in this PR.  If users have merge conflicts they should be minimal.
I added the jekyll-date-format gem which does this in a less intrusive fashion. Octopress Ink has this stuff built in and does a better job overall, so this plugin will eventually not be needed.