allow setting the date when creating a new post

This is the start of being able to pre-date blog posts.
do not build posts whose day has not yet come

Now we should be able to make a nightly job to run "statocles deploy"
and blog posts will show up correctly.

Fixes #8
add paginate() constructor for paginated lists

The paginate constructor returns multiple Statocles::Page::List objects
based on an input set of pages.
add optional index path to pagination

This makes building the blog paginations a lot easier. Instead of having
to fix the URLs after they've been made, they get made correctly in the
first place.
enforce an order on the paginated pages

We need to know which comes first in order to set our site index page.
add pagination to the blog

The index page and the tag pages are paginated. The index page is now
the first page returned from index() (though this may need to be
changed, we are generating pages repetitively).

Fixes #9
fix the pagination links in the default blog theme