Fallback to targeted ID

Strathclyde, Surrey and possibly other institutions were not passing an
eppn or a persistent ID in the Shibboleth response, so added fallback
to targeted-id if both of those are absent
Merge pull request #78 from DigitalCurationCentre/Shibboleth-auth-bug

Merging myself due to no-one else being around today and the long weekend coming up.