Add support for multiple AssertionConsumerService endpoints.

This patch adds support for multiple AssertionConsumerService endpoints
on the IdP, so that a SP can request that the response is sent to a
specific ACS URL.

Thanks to Georg Gollmann for this patch.

Fixes issue 20

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1815 44740490-163a-0410-bde0-09ae8108e29a