Added a Warning about installing Oscar on Windows

Several of Oscar dependencies don't support Windows, some of them don't
work at all. Added a warning that indicates installing Oscar on Windows
is a tricky process.
Remove alert that is irrelevant to the user

This alert is potentially confusing noise for sites that sell only downloadable
products. I couldn't think of a case where this alert is necessary for sites
that sometimes require a shipping address and sometimes don't.
Added `group_pricing_policy` to `FixedRateTax`

Using a strategy derived from the `FixedRateTax` mixin breaks the default templates (which require a `group_pricing_policy` attribute).  I basically copied the one from `NoTax` and added the tax calculation from `FixedRateTax.pricing_policy`.
Enhancement of product_saved.html template

Replace url tag to product get_absolute_url method
Docs: updated info about Core shipping methods
Fix image preview in dashboard

The JS had the wrong element IDs.

Fixes #1325
Merge pull request #1328 from dimka665/patch-2

Docs: updated info about Core shipping methods
Merge pull request #1327 from dimka665/patch-1

Enhancement of product_saved.html template
Merge pull request #1321 from machtfit/checkout-remove-noise

Remove alert that is irrelevant to the user
Merge pull request #1304 from mmardini/patch-2

Added a Warning about installing Oscar on Windows
Add request.basket_hash for custom caching hooks

To add caching to the basket contents we need a basket hash but we don't
want to load and process all offers. Separate the retrieval of the
basket and make the hash accessible via request.basket_hash
Add comments to better explain basket middleware
Remove unnecessary SQL queries

- Check FK existence in a few places by looking for the ID directly
- Use basket's all_lines method to re-use cached basket when counting
  lines.
Rework basket middleware

- Extract caching logic into one place
- Re-order methods for clarity
- Rename things
Update v0.8 release notes with middleware change
Ensure vouchers are moved to new basket when merging

This solves an issue where a customer's voucher is dropped when they
sign in.

Fixes #1187
Merge pull request #1324 from mexicat/patch-1

Added `group_pricing_policy` to `FixedRateTax`
Improve the recipe on supporting US taxes
Ensure all pricing mixing have a group_pricing_policy method
Add US-specific demo site

This simple site illustrates how to use Oscar for a US-specific site.
It's intended to help developers build Oscar sites for the US.

It can be built using:

    $ make us_site

This will build the database and load some sample products.

The key changes from core Oscar are:

- Setting the default currency to USD in settings.
- Overriding Oscar's partner app so we can specify out own Selector
  class which returns the US strategy. This ensures all prices are shown
  without tax.
- Overriding the checkout session mixin so we can apply taxes to the
  submission and also pass a template variable that specifies taxes
  should be broken out separately. We use a dummy tax module that
  applies very simple taxes to the submission.
- Overriding the checkout shipping address form so we can force the
  state field to use the US state dropdown.