Fix for #1129

serializers.md: Fix typos

Fix URL/Slug Field signatures in docs

fix docstring typos.

Use www.django-rest-framework.org for docs instead of django-rest-framework.org due to issues with naked domains

Use www.django-rest-framework.org for docs instead of django-rest-framework.org due to issues with naked domains

FIX BaseSerializer.from_native has an altered signature

Fix bugfix note

* base classes define it with one parameter
* BaseSerializer currently defines a second parameter, which we make optional here for method-dispatch passivity

FIx link to tox

This fixes a bugfix note that was added because of #1293, which
pointed out that a change in a bugfix [1] broke backwards
compatibility.  The bugfix did not work as expected because a
variable was quoted when it should not have been.  This removes
the quotes around the variable, which brings back the expected
functionality.

saving nested generic relation serializers broken in django 1.6

Simplified some examples in tutorial