Call signal receivers from models.py (not __init__.py)

This reverts fa1f8403 which moved them from models.py to __init__.py.
As reported by @mvantellingen, importing receivers from __init__.py
breaks the way models are normally overridden as it causes the core
models.py to get imported before the custom one.

We need to move back to the old system and will need to advise that the
debug toolbar needs to be set-up explicitly to avoid circular import
issues with models.

Fixes #1159
Original issue #1125
Handle group products when extracting price for search backend

Previously, the `prepare_price` method only considered standalong
products.  This changes adds handling for groups.

Fixes #1157
Use Oscar's AbstractUser model for the test suite

Previously, the test suite ran with a custom user model for Django >=
1.5, but it didn't subclass Oscar's AbstractUser.  This commit adjusts
the custom user model to subclass Oscar's so we can test functionality
within the custom user class.

This does require a change to the way tests authenticate as we don't
actually use the username field.
Adjust import of `StockAlert` to avoid circular reference

We now use the related manager to access the model class rather than
importing at runtime which causes a circular reference.

Fixes issues reported in comments of #1127
Bump version to 0.6.2

Plus add some release notes.
Config examples on additional facets with 'show_zero'.
remove Duplicated MultiImage variable.
Merge branch 'releases/0.6'

Merged after 0.6.2

Conflicts:
	docs/source/releases/index.rst
	oscar/__init__.py
	oscar/templates/oscar/customer/wishlists/wishlists_detail.html
	oscar/test/testcases.py
	tests/functional/dashboard/order_tests.py
Remove sandbox partner migrations

They're not required and we're committed accidentally.
Add link to django-oscar-jobs mailing list to README

We might have to give it visibility in the docs as well, but I did not
find an appropriate place.
Merge pull request #1162 from astyfx/patch-1

remove duplicated MultiImage variable import
Revert relative imports for non-module sandbox settings

c06e1df wrongly changed the imports to relative imports, as settings
isn't a module (but probably should be). Reverting those changes for
now.
Drop unused ProductCategory.is_canonical field

The BooleanField is not used in Oscar core, and not commonly required.
The through-model ProductCategory is empty apart from the two
ForeignKeys, but left in place to allow customising.
Fix for get_classes when used with non-Oscar modules

get_classes allows imported_oscar_module to be None.
imported_oscar_module ends up in modules in _pluck_classes, however the
code is not prepared to handle None at that place.

Fixes #1160.

Thanks to @mberthau for reporting and supplying fix.
Fix two translation string issues

The forms string is a duplicate of another string apart from the dot.

The regexp error text was a bit too techy.

Thanks to OdifYltsaeb for reporting.
Optionally disable including salutation in active_address_fields

I think it's a common enough requirement to justify the extra line of
code. Requested in #1156.

Fixes #1156.
Merge branch 'master' of https://github.com/tangentlabs/django-oscar into feature/demo-multipayment-with-faceted-catalogue
Fix ProgrammingError "relation does not exists" during validate process. Model initialization avoided at compile time.