Add Australian Company Number validator

[koterpillar]

Add Australian Company Number model and form fields.

Note that running isort --recursive --line-width 120 localflavor tests produced changes in otherwise unchanged files. I've limited my changes to the files I touched.

All Changes

• [x] Add an entry to the docs/changelog.rst describing the change.

• [x] Add an entry for your name in the docs/authors.rst file if it's not already there.

• [x] Adjust your imports to a standard form by running this command:

  isort --recursive --line-width 120 localflavor tests

New Fields Only

• [x] Prefix the country code to all fields.

• [x] Field names should be easily understood by developers from the target localflavor country. This means that English translations are usually not the best name unless it's for something standard like postal code, tax / VAT ID etc.

• [x] Prefer 'PostalCodeField' for postal codes as it's international English; ZipCode is a term specific to the United States postal system.

• [x] Add meaningful tests. 100% test coverage is not required but all validation edge cases should be covered.

• [x] Add .. versionadded:: <next-version> comment markers to new localflavors.

• [x] Add documentation for all fields.

[koterpillar]

Fixed both issues.

[benkonrath]

It would be great if you can also add a test the form's prepare_value. I think there's an example test with the AUBusinessNumberField. Thanks.

[koterpillar]

Added tests.