Explicitly yeild tasks containing subtasks to add docs and remove ugly code

Note: this patch requires doit >= 0.23

Before doit 0.23 it was not possible to set the "doc" attribute of a task containing sub-tasks.
Now it is possible by yielding a task with "name" equal to None.

This also allows nikola to get rid of some ugly code that created a dummy task to make sure
all named tasks were present.

minor notes:

* I found a problem that "render_tags" was a creating a sub-task in "render_site". 
Fixed that and added an assert to make sure other Tasks wont misbehave.

* made render_site and post_render to be simple tasks instead of creating a single sub-task

Off topic: the name "post_render" is confusing. I was thinking it was related to a "post" in the blog. what about "after_render"?