[contrib.postgres] UUIDField

Added UUIDField. Not particularly complicated this one - no custom lookups or anything. Casts all values to `uuid.UUID` instances in python.

TODO:
- [x] More form field tests
- [x] Documentation

Squashed patch should be marked as fixing #19463.
Postges can create uuids, when installing the extension.
 Am 19.07.2014 15:20 schrieb "Matthew Schinckel" <notifications@github.com>:

> In django/contrib/postgres/fields/uuid_field.py:
>
> > @@ -0,0 +1,43 @@
> > +import uuid
> > +
> > +from django.contrib.postgres import forms
> > +from django.core.exceptions import ValidationError
> > +from django.db.models import Field, SubfieldBase
> > +from django.utils import six
> > +from django.utils.translation import ugettext_lazy as _
> > +
> > +
> > +class UUIDField(six.with_metaclass(SubfieldBase, Field)):
>
> Hmm. It occurs to me that if primary_key=True, and we do this, it would
> look like the object already exists in the database before it actually is
> saved.
>
> Although, since postgres cannot create UUIDs, that's going to present a
> problem anyway.
>
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> <https://github.com/django/django/pull/2923/files#r15143651>.
>