Improve performance of DynamicSiteMiddleware.

Instead of asking the database if a Site object has changed, cache the
original value and check it.

As well, pre-populate the SITE_CACHE when resolving aliases. Writing
to the cache too often is better than hitting the database.
Bump version to 0.2.4
Added tag version-0.2.4 for changeset b1cedca9137c
Cross-domain cookie support.

Prepend multisite.middleware.CookieDomainMiddleware to
settings.MIDDLEWARE_CLASSES to handle cross-domain cookies.
    http://en.wikipedia.org/wiki/HTTP_cookie#Domain_and_Path
Added tag version-0.3.0 for changeset ca16e31171a0