Fix CountryField's widget choices being empty after translation fix in #77

Fixes #81
Revert "Don't return a Country object for a null or empty string"

This reverts commit dc57acf8f1125b7aa15e59a560ca2b92abba8381.

Conflicts:
	django_countries/fields.py
	django_countries/tests/test_fields.py
Revert descriptor to always return a Country object again

While the change for #47 was reverted in the previous commit, this
should also make the requested behaviour work as well (country=Null
actually commits NULL).

Fixes #82
Release a bugfix version
Fix CountrySelectWidget failing when form created with a model instance

Fixes #86
Bugfix release for CountrySelectWidget
Tests for COUNTRIES_ONLY

Refs #80
Add COUNTRIES_FIRST option

Fixes #84
Add change log, listing backdated changes to v2.0

Fixes #78
Add Arabic translation