Document limitations of get_classes

I investigated for a few minutes if it's possible to get
get_class('shipping', 'Scales') working. oscar_module loads correctly,
but any overriding local_module was missing classes defined therein.

Just documenting this limitation explicitly instead of tripping at the
ValueError when trying to split.
Rename and move Scales class, and make it overridable

Making it overridable allows altering the weighing method of the basket,
which I wanted to do in a project. As get_classes doesn't support
importing from top-level products, it had to be moved to a scales
module.

Other class names tend to use singular, so as the imports had to be
changed anyway, Scales was renamed to Scale.
Tweaks for model-based shipping methods

* Made upper_limit a DecimalField, like the other weight fields
* Increased default precision for weights to be to the gram level. Will
  be useful for deployments shipping smaller items such as stickers,
  where 10 grams precision is too low.
* Changed strings to official 'kg' abbreviation.
Drop unused Javascript from demo site

Alex Moon ran into an issue which led to us grepping through the code
base and discovering some Javascript libs aren't used by the demo site.