Aggregation: sharedToken aggreg in doc and config.

Include documentation and sample config for aggregation by
auEduPersonSharedToken.

Note that this configuration uses the NameID format
''''urn:oasis:names:tc:SAML:2.0:nameid-format:persistent'''' to
represent the auEduPersonSharedToken value - and relies on the Groups
IdP mapping this back to a Principal name with the following
configuration line (also included in this commit):

    <resolver:PrincipalConnector xsi:type="pc:Direct" id="sharedToken" nameIDFormat="urn:oasis:names:tc:SAML:2.0:nameid-format:persistent"/>
Documentation: fix markup in simpleaggregation.md
Merge pull request #8 from University-of-Auckland/develop-aggregation-sharedToken

Aggregation: sharedToken aggreg in doc and config.