Added new  {TagsAsClasses} method for Posts 

I noticed that there wasn't an implementation for TagsAsClasses, which is a tag on Posts (http://www.tumblr.com/docs/en/custom_themes#posts) so I added a simple implementation and some tests for it.

Sorry for the messyness of these commits, had to fiddle with it a bit to get it all working!

Thanks for your awesome work on this gem. Was a real help to me.