Attributes: add missing optional attributes

Add attributes defined in AAF and Tuakiri vocabulary as
Optional/Recommended but so far missing in attribute validator:

* auEduPersonLegalName
* auEduPersonAffiliation
* eduPersonPrincipalName
* eduPersonPrimaryAffiliation

Add corresponding validation methods (but permit blank as these are
Optional attributes).

Add mappings in application_config - using the usual SP name of the
attributes.

Add corresponding validation error messages.

Add tests.

All tests passed.

Implements #7 (now that I see it exists as a recently created issue).
Attributes: update default mappings

Update default mappings for the authenticationMethod and
organizationalUnit attributes to use their usual SP name.
Merge pull request #16 from University-of-Auckland/develop-add-attributes

Add missing optional attributes