Layouts default to page or post.

This patch defaults layout for pages and posts to `page` and `post` respectively. In many cases (I suspect the vast majority) this is a perfect setup. And one need not worry about putting a layout entry in each markup file. This is especially useful to me using a Gollum wiki as the source of my posts b/c I can't add YAML front matter. So having default layouts makes all the difference.

This patch shouldn't have any adverse effect on previous designs since layout was effectively a mandatory field before.