Avoid using restrictions on our data in the database; let's design the
system to keep integrity in the app (and make the app robust against
minor integrity issues like duplicate tags, etc).
$theme->block_type() so that the themer has a consistent interface.
Also added a bunch more callbacks and normalized the names so that the
module author has plenty of options for where stuff gets put on the
page. Especially renamed album/photo/sidebar to be album_blocks()
photo_blocks() and sidebar_blocks() to make it clear that those are
going to be larger content sections and not just basic insertion
points.
Used __call() to collapse all functions in the theme, which
incidentally makes it trivially easy to add a new insertion point.
