Use strategy to look-up prices and availability

Fixes #881

Still needs more attention but this fixes the problems when running
"make demo".

The CSV importer is one of three equally broken ones in the repo.
Nonetheless, it's rather slow due to lack of not happening in a
transaction. This one-liner speeds it up by factor of two.

* Analogous to 64b59f the default cache for the demo site has been
  switched to LocMemCache instead of memcache
* Comments regarding usable databases have been updated
* The language code has been changed to en-gb

- Add setting for demo
- Correct typo for sandbox

Dependencies installed via setup.py stay in their eggs and their
templates won't be found by Django. This caused a
TemplateNotFoundError when users were trying to use
Django's admin to set categories.
Using the admin is unsupported, but this still prevents a similar error
from happening when we use other dependencies that ship templates. It
also shouldn't slow down production use.

Fixes #761

I used the existing script [1] that cooperates very nicely with vim.
1: https://github.com/rory/django-template-i18n-lint

Fixes #676

- Only handles Datacash payments at the moment

- Fix payment event reference issue
- Upgrade django-oscar-datacash
- Handle changes to bankcard obj

This is done by staticfiles' runserver command automatically.

* Emailbackend: Updated to use REQUIRED_FIELDS
* Demo and sandbox site now use compat.AUTH_USER_MODEL as well
* Added example custom user that gets tested for Django >= 1.5

So that it's possible to use a different URL for static assets.

Use blank instead of null.

Without this, South was choking as it couldn't find a 'default' key in
DATABASES.

They were copies of an old Django 1.3 project version with some Tangent
extensions.

To avoid having cache clashes.

The parameters have been changed to use request, not request.basket.

Their links were not clickable previously.

- Remove duplicate lists from footer.html.
- Remove "All" prefix from browsing as it doesn't make sense when the
  summary is "All products".