Typo fix in docs/api-guide/relations.md

`slug field` in code snippet was replaced to `slug_field`

don't import authtoken model until needed

Min/MaxValueValidator is no longer transferred from a model's DecimalField

These two tests were previously added in
https://github.com/tomchristie/django-rest-framework/commit/7d79cf35b7be01b175d8c25276a4414e8144a16b
but we have now discovered that there are not actually two separate
cases, there was just a  bug in the code that made it look that way.

This also removes a redundant check to see if `DecimalValidator` was
defined.

Make code block consistent in serialization tutorial

All the other code blocks in the Serialization tutorial can be copied and pasted, but there is one that includes the >>> shell prompt characters. This commit removes those characters, and also makes the output consistent with other code blocks by making it a comment.

Initial fixes #3636 and fixes #3637

Add HTTP status code 451 to status.py

This test was incorrectly checking that there were no validators set in
older versions of Django, even though it should have been checking for
the two validators that were set up on the model field level.

The originally regression test that this fixes was added in
https://github.com/tomchristie/django-rest-framework/commit/7d79cf35b7be01b175d8c25276a4414e8144a16b
when fixing an issue with the `DecimalValidator`.

Previously, all validators set on a DecimalField in Django would be
stripped when converted to a Django REST framework field. This was
because any validator that was an instance of `DecimalValidator` would
be removed, and when `DecimalValidator` wasn't supported (so it was
`None`), all validators would be removed.

This fixes the issue by only removing the `DecimalValidator` instances
if the `DecimalValidator` is supported.

This adds tests for a regression where the `min_value` and `max_value`
arguments are not being set for a DRF `DecimalField` even though the
corresponding `MinValueValidator` and `MaxValueValidator` is being set
on the model fields.

Note that this only appears to be a regression for Django < 1.9, as
these regression tests pass on newer versions of Django.

The Internet Engineering Task Force approved the new HTTP status code 451. Adding to status.py and docs appropriately.

Fixes #3756, adding the name to the HTML title, if it exists.

Update the Django-guardian version for Django 1.9

Add DRF docs to "Documenting your API"

Update bootstrap version in browsable-api.md

Issue 3726 DateTimeField not handling empty values

Raise error when setting a removed rest_framework setting for #3644

Subclass AutoFilterSet from self.default_filter_set