get_available_shipping_methods must always return an iterable.

* Add note about ``get_default_shipping_method`` shipping repository method to the documentation.
* Add note how to add custom node to the dashboard navigation with non-dashboard URLconf.

Add docstrings to the following classes in 'oscar.apps.shipping.methods':
* Free
* FixedPrice
* TaxExclusiveOfferDiscount
* TaxInclusiveOfferDiscount

Fix typo in 'docs/source/howto/how_to_configure_shipping.rst':
* Change 'shipping.addr' to 'shipping_addr'

The howto recipe on shipping has been rewritten to describe the new
changes to shipping functionality. The old shipping app reference has
been merged in too to avoid duplication.

These days, to override a mixin it is only required to override the
OrderPlacementMixin; overriding the app and view is not necessary. It is
just a generic case of overriding a core class. A dedicated how-to is
thus unnecessary.

As overriding a core class is the general goal of any advanced
customisation, it has been merged into the customisation section, which
has been given an overhaul as well.

The section regarding modifying apps will be re-added as part of a
higher-level "the Oscar application tree" kind of document.

Changing a unofficially deprecated model name is clearer than having
three places to get at the same model.

The name 'AbstractBase' is consistent both with our naming of other
abstract models, and lots of apps having a Base class.

This is a common and useful Oscar pattern, and there's no reason to not
use it for the shipping app.

They are not imported anywhere in Oscar and just provided for
convenience, so no imports had to be altered.

Underscore does not seem to have meaning in Sphinx, was rendered as-is.

Now using Napoleon :)

To set a base for upcoming docs changes, the docs directory structure
has been streamlined to follow a simplified version of Django's docs.

The old _draft folder has been removed. Both files from there and the
current docs that don't fit nicely into the new structure are in _old/
for the moment.