Fixed {% render_plugin %} example

The instance.child_plugins reference is not correct, it should be instance.child_plugin_instances.
Merge pull request #2045 from andreiavram/patch-1

Fixed {% render_plugin %} example
Merge branch 'develop' of ssh://github.com/divio/django-cms into develop
remove relative positioning from item
this fixes additional issues with collapsable positioning saving
adds initial esc command to the modal window
Merge pull request #2108 from FinalAngel/feature/fixes

Additional Frontend Fixes
Document the required messages framework
Merge pull request #2119 from yakky/feature/docs_messages

Document the required messages framework
Merge remote-tracking branch 'upstream/master' into develop
Merge remote-tracking branch 'upstream/develop' into develop