Created base templates for email html and txt layouts in order to avoid repeating layout accross files.
Add `autocomplete=off` to dashboard product form

Fixes #631
Docs: Refactor translation howto

The old howto included out-dated info on how to translate Oscar. That
has been replaced by a link to the Transifex project.
It was the only howto left under the Contributing headline, which really
shouldn't be there (as there's an entirely different section in the docs
for contributing). As it is more of a topic anyway, and will hopefully
soon include documentation on translation style etc., it was turned into
a topic.
Docs: Customisation topic to tie-in how-tos

Attempt at making it clearer what needs to be done for what kind of
customisation. Created a new topic that lines out the prepatory
steps that need to be taken, and moved duplicate information from the
how-tos into the new topic.
Docs: Fix broken sandbox link
Docs: Remove empty how_to_model_your_catalogue

It's a great idea, but confusing when empty. Replaced by links to
how_to_customise_models which contains some explanations.
Started injecting request object into AddToBasketForm
Email templates refactored: extend from base template.
Injecting request object into AddToBasketForm

It was injecting user and basket separately before onto form contructor.
The result was that in some cases it would make it difficult to twist
functionality that was depending for instance on the request object.

Passing the whole request object on the form will make it more flexible
to change.

Also, the basket_tags where reviewed they also started passing the
request object instead of the user.
Template tags calls form permission

This commit will make the templatetags to pull the form rendering
permission from the form object.

Also there will be two attributes set in the form when redering
purchase_permitted, and reason. This will allow a better information to
be displayed on the template.
Update context_processors.py

Fixes issue with other modules where a request without a user might exist and cause that module to fail.
Improve docs on how to customise an app

- Made instructions on using a custom template more expliciy.  Should be
  better for new users.
- Fixes a few typos and mistakes in the code examples.

This commit is squash of pull request #640.
Merge pull request #641 from pbrooks/patch-1

Update context_processors.py
Extend release notes with jQuery upgrade
[Backport] Fix bug with product class lookups in variants

This causes availability messages to fail in templates as an exception
is raised for variant products which don't define a product class.

Cherry-pick of 72eaafecb356032d198248e485a0fbcdfad5e3f2 back onto
releases branch.
[Backport] Handle group products in stockrecord template partial

Before this, group products were always shown as unavailable as they
didn't have a stockrecord - even though their variant products did.

Cherry-pick of f2922c9b3a63f035e20efae99a9fd3ba2e5c3b6d.
[Backport] Correct product URL on thank-you page

Cherry-pick of af6edf88ea419a4dc2e409ea61686f645c3ec19c
Release note for 0.5.1
Reinstante xunit file generation for 'ci' make target
Add details of Tangent's support offering to README

Also bring logos into repo (rather than imgshack).