Added rudimentary suppport for the Scoping element i saml2.

Scoping allows a sp to give a proxy/bridge idp a list of (backend)idps.
The list can be given as parameters to InitSSO or/and in metadata for
the sp-hosted or/and the sp-remote.


git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1772 44740490-163a-0410-bde0-09ae8108e29a