Build out Cell baking.

Extend SimpleBakeTask to generate the class, test and template. I still
need to finish off the TestTask changes.
Clean up doc blocks and __toString() / render()

Code blocks need to render correctly in the API docs, and I prefer magic
methods to not do the bulk of the work.
Remove an unused method.

This method never gets called, so it can be removed.
Implement test generation for cell classes.
Fix failing tests.
execute() was renamed to main() on tasks.

Reflect changes in 3.0 since this work started.
Merge pull request #3393 from markstory/3.0-cell-bake

3.0 cell bake