Introduce loud get_model loader

Django's get_model returns None if the import fails, which causes subtle
bugs further down the line. This change introduces a wrapper around it
which raises an ImportError instead, hence catching the error as early
as possible.
Switch to Oscar's get_model function

This should always be used instead of Django's silent version.
Fix ImportError highlighted by switch to loud get_model loader

forms.py was using the wrong import statement to load the User model.
[Backport] Mark more strings translatable

Fixes #1171

(cherry picked from commit 0a72bf27c93322a10afbda602e87935438272552)
[Backport] Correct handling of offer creation without a range

The workaround for JSON serialisation introduced a bug that was
triggered when the offer contained no range, which is allowed for
certain offer types. In that case range is None, and accessing range.id
threw an error.

Fixes #1167. Thanks to @izidormatusov for reporting.

(cherry picked from commit 1e85821d85068c71d7d5b959fce743f1b7bfe248)
Fix issue with custom user model in order migration
Make template for checkout nav/progress indicator more readable

It was rather tricky to get a grasp of what's going on in that template,
so it's been more sanely indented. The code is still rather verbose, but
that can be left for another time.

Only code change was correcting a <span> in step 3 which was outside the
{% if %} instead of in it.
Fixed change_order_status in dashboard's OrderDetailView.
Old status was not shown after changing. Status needs to be saved before
order will be passed to handler.
Extend docstring for AbstractProduct.status

Done because a question on the mailing list arised about the use of it.
Merge pull request #1185 from vovanbo/fix/dashboard-change-order-status

Old status of order was not shown after changing in dashboard
Positional argument specifiers are needed for python 2.6

http://docs.python.org/2/library/string.html#format-string-syntax
Fix migration compatibility with Python 2.6

@mbertheau provided a fix in #1176. That commit has been applied to
master. This is fixing the same issue for Oscar 0.6
Mark some strings translatable
Add release notes for 0.6.3
Ensure order number searches don't error

Fixes #1186
Update release notes
Fix lint error
Update translations for 0.6.3
Bump version to 0.6.3
Merge branch 'releases/0.6'

After 0.6.3 release.

Conflicts:
	docs/source/releases/index.rst
	oscar/__init__.py
	oscar/apps/order/migrations/0019_auto__chg_field_order_billing_address__chg_field_order_user__chg_field.py
	oscar/apps/promotions/models.py