Better handling of multi-valued attributes

This commit modifies the behaviour of getAttributeValue() so that when
multiple values are present in the CAS token, only the first is
returned. This is useful mostly to handle multiple email addresses.
Merge remote-tracking branch 'mabes/multiattr'
Changed 'Try again' link in failed login page to be relative instead of
absolute (fixes issue when Jenkins is run from sub-uri).
Update license header dates.
Create README.md
Update README.md