Allow free placement of posts.

This patch allows posts files to be placed anywhere, not just in _posts directory. Instead of looking only in _posts, it selects all files with a prefixed date in their name. So things will still work fine for sites that use _posts.

This is helpful in my case b/c my posts are being ported from my Gollum wiki. If I had to put them in _posts I'd have to do additional staging and copying of files. After thinking about it, I realized that there wasn't any real barrier to allowing Jekyll to handle the free placement of posts, and really this simplifies the design a bit in general which is always nice.