Applied patch described in https://github.com/toastdriven/django-tastypie/issues/526
Expanded on the documentation
Added tests for optional fields feature
Added the ability to make the use_in field a callable
Refactored the conditional a little bit to make it more clear
Added tests for use_in as a callable
Make use_in available to related fields

Conflicts:

	tastypie/fields.py

Pass use_in to the superclass.

Conflicts:

	tastypie/fields.py

Tests for related resources
Refactored code for setting default properties based on feedback by @toastdriven
Added the appropriate authors
Updated docs a little to fix some typos and clarify callables