Purge naked excepts.

Most of these had obvious exceptions which would be thrown. Some I'm not
sure about but they should at least catch only Exception so as not to
ignore SystemExit and other inappropriate Error classes.

Fixes #639.

2.2 Release

Need to track outcome of Django's #19733, and decide on approach
accordingly.

Mostly adding `from __future__ import unicode_literals` everywhere.

Conflicts:
	rest_framework/relations.py
	rest_framework/serializers.py
	rest_framework/tests/relations_hyperlink.py
	rest_framework/tests/relations_slug.py

(instead of request.GET)

Commiter number 100!

Conflicts:
	rest_framework/authentication.py
	rest_framework/relations.py
	rest_framework/serializers.py
	rest_framework/settings.py
	rest_framework/tests/authentication.py
	rest_framework/tests/genericrelations.py
	rest_framework/tests/generics.py
	rest_framework/tests/relations_hyperlink.py
	rest_framework/tests/relations_nested.py
	rest_framework/tests/relations_pk.py
	rest_framework/tests/serializer.py

Conflicts:
	rest_framework/relations.py