Fix missing content blocks for cached smartsnippets.

The issue fixed is the restoring in the context object of the Sekizai
content. Normally the content would have been added at the time
the *addtoblock* tag is rendered. Unfortunately this caused missing
css/js content since the addtoblock tag never got rendered. Therefore
the blocks content never got added in the context object.

Sekizai 0.7.0 provides a watch mechanism for the context objects.
This keeps track of what's being added to the context after rendering.
By storing alongside the rendered content, the this context difference
we can restore the missing *addtoblock* contents.

See also: https://github.com/divio/django-cms/issues/1577
Add tests to smartsnippet caching restore.
Add cache invalidation on save.
Delete cache keys in one go.
Merge pull request #42 from pbs/restore_caching

Fix missing content blocks for cached smartsnippets.