Refactor UpdateModelMixin.update

Reduce nesting, return early in error cases.
Remove unnecessary else
Merge pull request #1427 from Ian-Foote/refactor_update

Refactor UpdateModelMixin.update
Tweak comment wrapping.
fix(docs): fix code indentation in serializers.md

It fixes the indentation of `restore_object()` in the section "Specifying which fields should be write-only" serializers documentation.
Merge pull request #1428 from theskumar/patch-1

fix(docs): fix code indentation in serializers.md
include djangorestframework-httpsignature in the docs
Merge pull request #1433 from etoccalino/http-signature

include djangorestframework-httpsignature in the docs
Link to DRF-extensions. Refs #1430
Minor docs example fix

Dates are smaller when they happen before.
Merge pull request #1436 from meshy/patch-1

Minor docs example fix
Demonstrate problem post/put'ing serialized data that contains 'None'
Display the media type of the API response on the browsable API
Update Travis to test Django 1.6.2

We should be testing the latest version of Django 1.6 as we're doing in the other 1.x releases.
Allow 'None' to pass as a null value in RelatedFields
handle negative time value and prevent a divide by zero
Merge pull request #1440 from un33k/master

handle zero and negative available_requests
Merge pull request #1441 from dustinfarris/null-serialization

TestCases and POST/PUT'ing serialized data that contains `None`
Merge pull request #1443 from silent1mezzo/patch-1

Update Travis to test Django 1.6.2
Merge pull request #1437 from Keats/master

Display the media type of the API response on the browsable API