Enable autodoc sphinx extension. Document PageQuerySet

This enables a quite useful module of sphinx called autodoc which builds documentation based on the python docstrings on the classes.

I've made use of this for documenting the Page and PageQuerySet classes.

The autodoc extension needs to import modules to find docstrings but Django gets unhappy if you import any models.py files without settings being configured first. To solve this, I've moved the test settings to ```wagtail.tests.settings``` and using that.