Added ability to add and delete posts.

Quite a few changes in this commit. Database changes were made; the
recordings table has been changed to riffs, and each riff is attached to
a post. The riffs have an .mp3 file associated with them in
SITE_ROOT/riffs/riff_id.mp3. These are optionally uploaded, with a
title, along with each post. You can also delete posts by passing the
post id, and the riff.mp3 file will be unlinked as well.

New classes, Post and Riff, have been created for posts/riffs. If a post
does not have a riff associated with it, $post->riff will be 0. Post
also has a User class associated with it.

Config has been changed to have a RIFF_ABSOLUTE_PATH for copying and
unlinking files.

The User class now also has date_created.