add support in core IDP for association groups which are used to share...

add support in core IDP for association groups which are used to share associations between SAML 2 and ADFS IDPs, to enable cross-protocol single-logout (thanks Olav)

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2181 44740490-163a-0410-bde0-09ae8108e29a