set Zip/PostCode not to be required

Some countries, ie. Ireland, don't have post codes, so:
1) post code should not be a required field
2) maybe conditional validation making PostCode required for some countries
Add setting to make address fields required

OSCAR_REQUIRED_ADDRESS_FIELDS allows setting which fields are required.
The use case is that post codes are expected to be required, but many
countries don't actually use them.
Merge pull request #583