Fix for <form> action attribute.

Old behavior is to default to action="." if no action is given. The dot
makes the browser submit to the URL specified, but drops any GET
parameters if the form is POSTed.
A common case of mixing POST and GET parameters is supplying ?_popup=1
to a popup. The view processes the form data in POST as usual, but then
might return a different HTTPResponse if 'popup' was supplied, e.g.
return Javascript to close the popup.

My fix drops specifying 'action' if none is given. This is okay in
HTML5 and HTML3, and defaults to the URL (including GET parameters).
It's required in HTML4, but as far as I know handled gracefully in
all browsers.
Test against multiple Django versions

Adds support for tox to test locally.
Adds support for Travis.
Update docs/ structure

To set a base for upcoming docs changes, the docs directory structure
has been streamlined to follow a simplified version of Django's docs.

The old _draft folder has been removed. Both files from there and the
current docs that don't fit nicely into the new structure are in _old/
for the moment.
Docs: Explain docs/ directory structure
Changing import of Image

The::

    import Image as PImage

was failing since the switch to `Pillow`. The commit changes it to::

    from PIL import Image
Update versions of haystack and virtual-node
Manual merge branch 'feature/betterdocs'

Conflicts:
	docs/source/recipes.rst
Merge pull request #514 from maikhoepfel/fix/formaction

Fix for <form> action attribute.
Replace pyflakes with flake8

Had the dependencies the wrong way around.
Normalise discount descriptions

Changed all occurences like 'discount is a % of the product's value'
to 'discount is a percentage off of the products value'.

Uses 'off of' instead of the semantically similar 'of' because otherwise
'Percentage off of products in range' would change meaning.
Merge pull request #474

Conflicts:
	requirements.txt
Fix testing conflict

spec==0.10 uses an unstable version of nose that I don't really want to
couple Oscar to right now.  So I've kept spec at 0.9.7 (which requires
nose<1.2) and nose-progressive<0.4 so that we can use a compatiable nose
version.

Related to second point of #578
Merge branch 'master' of github.com:tangentlabs/django-oscar
Add sandbox to build matrix

Conflicts:
	.travis.yml

Cherry-picked from #578