migrations: update after setting unique=True for blog title slugs
base_blog.html: surround placeholder text in <p> tags.

This will stack the elements vertically in stead of horizontally all in
a row.  It looks a little nicer.
GenericArchiveViews: set allow_empty=True to prevent 404

When there are no postings falling within a given date return a page
that indicates there are no postings instead of failing with a 404.
Add some very basic content to post_archive.html

List (date, title) for all blog postings.
Extract common generic archive view settings into a common base class.
templates: display a friendly message when there are no posts in the archive.
templates: flesh out the yearly archive view
tests: extract a get_archive_url helper method
templates: flesh out the monthly archive view
templates: flesh out the daily archive view
tests: add tests for empty and non empty post views
views: add the month_format field to BlogArchiveBaseView
views: add a basic detail view for a blog posting
models: add a get_url() function to the Post model

This function reverses using the name of the detail view that was added
in the previous commit.
templates: add a very basic 404.html template
tests: add a test for the detailed view of a blog post
templates: add links to blog postings from the archive views
tests: update get_archive_url() to use url reversing
templates: add links to archive pages
tests: add tests to check for the archive links just added

I installed the MonthDelta package in the process.