Improve handling of 'empty' values for ChoiceField

The empty value defaults back to '' (for backwards-compatibility) but
is changed automatically to None for ModelSerializers if the `null`
property is set on the db field.

Updated OAuth2 authentication docs.

Fixed exception handling with YAML and XML parsers.

Great. Thanks!

Docs: Fix for API test client example

need to define 'expected' as we can just test against the dict.

Removed commented-out credits from footer so that django-debug-toolbar works

The comment, although valid, caused that the Django debug toolbar's injected HTML was partially commented-out and thus the toolbar didn't work as expected.

Added get_filter_backends method

Typo on generic-views.md

Fixed UnicodeEncodeError when POST JSON via web interface; added test

Filter documentation explanation

Well, I spent some time trying to gues how djang-filter works, and
if this changes would be introduced, I would have saved this time.

paginator should validate page and provide default

better way comming in a seperate pr

Remove the detail=None from APIException signature

The documentation not match with the implementation. The APIException doesn't have detail parameter in the constructor class, actually doesn't have constructor method at all.

- use the standard paginator.validate_number method rather
  strict_postive_int.
- support optional paginator method, default_page_number, to get the default
  page number rather than hard-coding it to 1
- this allows supporting non-integer based pagination which can be an
  important performance tweak on extermely large datasets or high request
  loads
- relatively thorough unit tests of the changes