Update 2.4 docs url

Point to version 2 docs instead of the latest and greatest docs.

Backport 7872d0ac to 2.4.x

Fixed get_component method in Field to get working with subclassess of collections.Mapping

Set up serializer fields lazily on-demand.

Fix follow does not work on get of APIRequestFactory

Frameworks throws AssertionError saying you cannot set required=True and...

docs(api-guide/authentication): updated to reflect new standards in Django 1.7

Based on the [new documentation](https://docs.djangoproject.com/en/dev/topics/auth/customizing/#django.contrib.auth.get_user_model) for Django 1.7 we should now use:

```python
@receiver(post_save, sender=settings.AUTH_USER_MODEL)
```
instead of:

```python 
@receiver(post_save, sender=get_user_model())
```
because `get_user_model()` only works once Django has imported all models. Otherwise you'll get: 

```python
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.
```
When trying to start your sever after adding token authentication.

From the Docs, ( linked above as well ) :
>New in Django 1.7:
>When connecting to signals sent by the User model, you should specify the custom model using the AUTH_USER_MODEL setting.

Use PYTHONDONTWRITEBYTECODE=1 on tox environment

This fixes bad marshal data errors after running
tests with tox and later running tests manually
via runtests.py. Fixes #1957

Maintain order of views on router for api root view.

wider compatability.

This avoids AppRegistryNotReady problems in Django 1.7 with nested serializers,
which are instantiated at import time, possibly before Django's app registry is
fully populated.

Add docs regarding linking third party packages

Add djoser to authentication docs

Handle follow just like Django's Client.

Update links in 2.4-announcement.md