Tutorial: fix module name in section 3

Tutorial: fix module name in section 2

Again snippet -> snippets, but then it could be simpler (and possibly intended) to rename snippets to snippet in the first section of the tutorial.

snippet -> snippets (to match section 1).

Cache all fields per serializer instance

Add widgets for DateField and DateTimeField

Add support for partial serializer updates

http://www.python.org/dev/peps/pep-0008/#programming-recommendations

minor docs fix

http://www.python.org/dev/peps/pep-0008/#programming-recommendations

added RegexField

Conflicts:
	docs/topics/release-notes.md

Serializer method field

Cache default_fields per serializer instance for faster serialization