Added test for sekizai/show-placeholder issue
_show_placeholder_for_page to also cache sekizai changes

Fixes #1577
Merge branch 'develop' of github.com:divio/django-cms into fix-show-placeholder-sekizai
use django TestCase for assertIn
don't call get_varname that often
further micro-optimize the _restore_sekizai function
Merge pull request #1596 from ojii/fix-show-placeholder-sekizai

Fix show placeholder sekizai