Disable caching for "everything till now"
Add a cache handler for the archive for current items

This enables caching for the archive for the current month but makes
sure that the cache is always current by expiring the cache whenever a
page is changed or a comment or linkback is created.

Note that the cache still doesn't respect ACLs.
Ensure that the archive doesn't list pages the user can't access

This makes sure that the cache for archive pages is never used when a
page is listed that the current user can't access. For simplicity and
speed reasons the opposite check, i.e. if pages are missing that the
current user could access, is missing. It is assumed that this isn't the
typical use case for this plugin, a simple workaround is using the
~~NOCACHE~~ macro in the page.
New version 2013-03-27

Changes: Better cache handling for the archive syntax
- the syntax that lists everything till now correctly updates the cache
  now whenever a page is changed or a comment or linkback is added.
- the cache is not used anymore when it contains an archive listing with
  a page that the current user can't access. The excluded pages won't be
  included again automatically unless the cache expires for other
  reasons, the ~~NOCACHE~~ macro can be used if this is needed.