six is presumably better tested than our own code. Plus it allows us
using both urlencode and urlparse.

This was meant to be added in 0.7, but forgotten. Now, the Python,
Django and Oscar version in use are reported if OSCAR_TRACKING is
enabled. This helps Tangent to make informed decisions about what
versions of Python and Django to support, and how long to maintain older
Oscar versions.

Reasoning about e.g. when it is feasible to drop Python 2.6 or Django
1.5 support is hard without reliable data, hence the tracker pixel has
been extended to submit the Python and Django version in use. Tracking
is still easily disabled by setting OSCAR_TRACKING to False.

This required:
* Moving Django's i18n urls into the main urls.py and outside of the
  scope of Oscar's main application class, because they need to live
  outside of the scope of i18n_patterns
* Add a language-code stripped version of the current URL to the
  template context to get the language switcher working.

I also updated the language switcher code to mimic what is currently
given as an example in Django's docs.

Fixes #797

Allow using a different app than promotions as the homepage view.

This is to enable some coarse metrics of who is using Oscar.  We just
look at the referer header to get a rough idea of which sites are using
Oscar.

This is an important measure for Tangent, Oscar's sponsor.  It's
documented in the release notes and we'll make sure everyone knows when
Oscar is released.  It's easy to opt out of.

Fixes #656

You only want to compile Less files if you are developing the CSS of
Oscar.  Most people don't do this and so we default to serving
compressed CSS without Less compilation.  This also means that
the many contributors don't need to bother installing node.js and lessc,
which makes the installation process much simpler.

This change also re-introduces the `USE_LESS` setting which controls
whether Less files are used or not.

This involves some changes to the template block structure to make it
easy to compress the correct files.

* A new block cdn_scripts has been introduced as CDN-loaded scripts
  can't be compressed.
* A new setting 'USE_LESS' controls whether to use the LESS-generated
  files or the compressed files (which will need to be generated
  manually as they aren't in source control).
* A fallback for jQuery UI is added
* The CSS block was renamed to be clearer
* The bootstrap CSS was brought out of the LESS file into the page.
  This makes it easier to compress.

To make it trivial to change the logo and branding without forking
templates.