Switched mimetype to content_type because of deprecation

The mimetype keyword was deprecated in Django 1.5,
https://docs.djangoproject.com/en/1.5/ref/request-response/#django.http.HttpResponse.__init__

The content_type keyword should be used instead.
Merge pull request #1488 from binarydud/mimetype_deprecated

Switched mimetype to content_type because of deprecation
Pin Webtest dependency

As 2.0.12 introduces a new feature which is used by an Oscar test.

We should really pin all these dependencies as using ranges is just
asking for trouble. I'll do that soon.
Fix tests for order status updating

The hidden widget means the tests have less set-up to do.
Correct typo in demo form meta
Whitespace
Install Django==1.7 from PyPi for Tox and Travis

We also drop the "allow_failures" section for Pythons 3.x on Travis.
They should pass consistently now.

I've also adjusted the 'travis' make target to not install Oscar so this
can be called separately as part of the Travis build. Hence, build
errors are reported differently from tests failing.
Remove whitespace from .travis.yml

Previous change has caused Travis to complain 'Please override the
script: key in your .travis.yml to run tests'.
https://travis-ci.org/tangentlabs/django-oscar/jobs/35319191

I am though so I'm confused. I did tinker with the whitespace last time
so here I adjust it in the hope that it fixes things. (I have no idea
what I'm doing).
Indent .travis.yml correctly

My Vim audo-indenting seems to have incorrectly put the 'script' key
under 'env'.
Add title to "sort_by"-field in search-form, and make it translatable
Warn about possible issues when running 'make sandbox'
Fix file upload process for updating ranges

Turns out this has been broken for ages. It was still assuming a
one-to-one relationship between products and stockrecords AND it had
broken translation strings in the flash message.

This commits fixes all the bad.
Merge pull request #1491 from yzzyx/translate_sort_by

Add title to "sort_by"-field in search-form, and make it translatable
Fix bug with moving saved lines back to basket

The old logic assumed that a basket's lines didn't contain duplicate
products. This isn't true when you have options.

Fixes #1496
Fix bug with moving lines from saved basket
Django 1.7 fix for payment models

Unfortunately, the payments app has been overlooked when adapting to
Django 1.7. Until this commit, it's not possible to override the payment
models.
Use string notation for factory boy factories

This avoids the imports triggering a population of the model registry,
and makes debugging model registration easier.
Remove unused customer/apps.py

This was accidentally added in 6503eb53, but is not used anywhere.
Update queryset manager method names

Django deprecated the old spelling a while ago, and warns aggressively
in Django 1.7. A simple rename is all it takes to fix.
Use get_registered_model to load User model in Django 1.7

In Django 1.7, get_user_model fails if the app cache isn't ready. As we
have to use it in many places in Oscar, this commit essentially loads
the User model like we load any other model in Oscar. This circumvents
the check for the model registry being ready.
Otherwise, there's major problems trying to avoid imports that trigger
the call to get_user_model (e.g. in forms and test factories) before the
first model is loaded (which then triggers Django's app load sequence).