Improve the subclass-ability of the RenderPlugin templatetag.
Merge pull request #3261 from divio/refactor_render_plugin_tag

Improve the subclass-ability of the RenderPlugin templatetag.
Ensure that __str__ really only returns text (unicode)
Add test to ensure moving or deleting AliasPlugins works
Merge pull request #3265 from divio/force_text_for_AliasPluginModel

FIxes #3264