Always show "download selected orders" option on order list view

There was no real need to hide it on the default view as you still had
to select some orders for it to work.

This also fixes #391 as the condition was assuming everything was
English.
Avoid divide-by-zero error when shipping discount is zero

Logic didn't cater for case where shipping charge was already zero.

Fixes #410.
Replace %% with %

This was showing up as '%%' in the offer dashboard.
Bump django-extra-views version

Fixes an issue in django-oscar-stores.  The bump is
backwards-compatible according to @andrewingram.
Allow the missing image class to be customised

The product class now has a method for returning the missing image
object.  Override this method to provide a custom implementation.

Fixes #406.
Offers are now active ON their end date

This is to make date range behaviour consistent across Oscar.
Vouchers are now active ON their end date
Trim fat from testcase

Use global instead of per-method test dates.
Split voucher testcase for better coherence
Adjust view logic to ensure end dates are treated inclusively
Fix queryset filtering for active offers
Refactor re-order view, moving logic onto Line model

This thins out the view slightly although my editor is still warning me
that it is 'too complex'.  Turns out there are quite a few things to
check when re-ordering a line.

Fixes #404.
Update nginx conf files for demo/sandbox

* Set expires=max for statics
* Add types for gzipping
Extract google analytics code into separate partials

So it can be reused on other templates when required.  Also, it makes it
easier to swap it out and use an alternative tracking urchin.

Fixes #399
Offers require a start date now

I've been convinced by Tangent's UX team that this is sensible.  The
main reason being that it makes it possible to determine when an offer
started (not necessarily the same as date_created).

Fixes #412
Add profiling middleware

These can be used to dump profile stats to the console during
development.
Make logout redirect non-permanent
Convert two redirects to use temporary codes

Don't think these two should be permanent.
Fixes test after changes in latest commit
Refer explicitly to virtualenvwrapper in docs

To avoid confusion.

Fixes #429