add method to clear document cache

Cache expiration is hard.

I'm tempted to disable caching in all of the low-level bits. If the
consumer wants to cache something, it can, but the Store should not
cache documents, and the App should not cache pages. Should the theme
cache?
add clear method to theme to clear template cache

When working on a site's theme, we want to be able to rebuild the
Template objects if they've been changed.
remove caching from Blog app

The caching is defeating my attempt to rebuild the site on the fly
(#154) and does not follow the caching rules for Apps (#153).

This is a performance hit, so we're going to remove the index() method
(#141).
remove index method from apps

The index page must now be the first page in the list of paged returned
by the pages() method of Apps. This reduces the number of API methods an
app must implement to (currently) one.

Fixes #141
Closes #142