Fixed #19617 -- Refactored Form metaclasses to support more inheritance scenarios.

Thanks apollo13, funkybob and mjtamlyn for the reviews.
Fixed #8620 -- Updated the Form metaclass to support excluding fields by shadowing them.
Fixed #21266 -- Fixed E201,E202 pep8 warnings.
Fixed #21223 -- Added form.id_for_label to topic guide.

Thanks ebertti at yahoo.com.br for the suggestion.
Fixed #7261 -- support for __html__ for library interoperability

The idea is that if an object implements __html__ which returns a string this is
used as HTML representation (eg: on escaping). If the object is a str or unicode
subclass and returns itself the object is a safe string type.

This is an updated patch based on jbalogh and ivank patches.
Fixed a webdesign template tag docstring to prevent parsing as metadata.

Previously admindocs would throw an error when processing it:
"Error in "default-role" directive: no content permitted."

refs #6681
Handle Apps with South migrations as unmigrated apps.
Merge pull request #1382 from loic/ticket19617

Fixed #19617 -- Refactored form metaclasses to support more inheritance scenarios.
Fixed #21006 -- Added example code for overriding methods on an InlineFormSet.

Thanks treysta at gmail.com for the suggestion.
Fixed #7603 -- Added a 'scheme' property to the HttpRequest object

`HttpRequest.scheme` is `https` if `settings.SECURE_PROXY_SSL_HEADER` is
appropriately set and falls back to `HttpRequest._get_scheme()` (a hook
for subclasses to implement) otherwise.

`WSGIRequest._get_scheme()` makes use of the `wsgi.url_scheme` WSGI
environ variable to determine the request scheme.

`HttpRequest.is_secure()` simply checks if `HttpRequest.scheme` is
`https`.

This provides a way to check the current scheme in templates, for example.
It also allows us to deal with other schemes.

Thanks nslater for the suggestion.
Fixed #21263 -- Fixed issue with override_settings in inherited classes

When both parent and child classes are decorated with override_settings,
child class settings should take precedence.
Thanks Sephi for the report and Marc Tamlyn for the review.
Improvement on InlineFormSet doc, refs #21006
Merge pull request #1751 from tmaster/ticket_21006_improvement

Improvement on InlineFormSet doc, refs #21006
Updated admin alert colors

refs #21220.
Moved misplaced import in backends init
Fixed #16855 -- select_related() chains as expected.

select_related('foo').select_related('bar') is now equivalent to
select_related('foo', 'bar').

Also reworded docs to recommend select_related(*fields) over select_related()
Made sqlite introspection also show views like other backends

Refs #6730.
Fixed DeprecationWarning caused by assertEquals.
Merge pull request #1755 from loic/typo

Fixed DeprecationWarning caused by assertEquals.
Merge pull request #1744 from unaizalakain/ticket_7261

Fixed #7261 -- support for __html__ for library interoperability