add 'apps' command to list the apps in a site

It also shows the URL root and the application class. This might be
subject to change in the future.
add application command delegation

Test it by creating a blog command help document. I don't like leaving
it inline like that, I'd prefer it be part of the POD, but this will
work for now.
add write_document method to Store

Now applications can edit their own documents
return the full path to the document

This is so we can notify the user where this document is in case they
want to know.
add 'blog post' command to add new posts to a blog

The blog post document gets initialized with some boilerplate and the
user gets told where it is.

Fixes #7