Add filename attribute to Post; clean up

Right now I'll pass in the filename manually when saving posts to the
store.  There are plenty of ways to come at this, a goal is to
eventually do all of this manual interaction through frontmatter
parsing.
Require post.rb in main app
Validate filename on post instantiation

Currently the filenames will be passed to the markdown parser, so the
yaml post data won't be of any use without them.
Add rake task for adding posts