Added a new file 'relations_slug.py' that tests Nullable Foreign Keys and the SlugRelatedField
Merge branch 'master' of https://github.com/steve-gregory/django-rest-framework into slug-field-fixes
Drop bits of relations_slug tests which don't mirror existing tests.
Added @steve-gregory for nullable slug relation tests.

See: #585
Updated release notes.
Note changes to Decimal rendering to json behavior. Fixes #582.