Feature/shared token

[rianniello]

The scope of this PR is:

• The retrieval and validation of values used for auEduPersonSharedToken generation.
• The generation of new auEduPersonSharedToken values.

Following this PR will be the database retrieval of auEduPersonSharedToken.

Some points to consider:

• In this implementation I've enforced a 16 character minimum for salt, which I've just read as a recommendation so far http://wiki.aaf.edu.au/tech-info/shared-token-install-guide.
• The original library https://github.com/University-of-Auckland/arcs-shibext/blob/master/INSTALL-SharedToken.md supports multiple values for the sourceAttributeId. This implementation only offers one value. See sourceAttributeId in this section https://github.com/University-of-Auckland/arcs-shibext/blob/master/INSTALL-SharedToken.md#configure-idp.

[rianniello]

I've just found that the idpIdentifier can be retrieved from the IdP itself (rather than needing to configure it outlined here https://github.com/ausaccessfed/aaf-shib-ext/commit/5f0e2eae9448c0834be1c60df084ab6d7e9dbfb6). Will fix this up now.

UPDATE: addressed here https://github.com/ausaccessfed/aaf-shib-ext/commit/52fdc0dbfdb3aa92e1cd7e1ea3d3832a4d0ff97d.