saml:SP: Add support for specifying bindings and required attributes in metadata.

This patch adds two enhancements to metadata generation:
 * Support for specifying attributes that are required.
 * Support for removing some of the AssertionConsumerService bindings.

Thanks to Benjamin Andresen for implementing this!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3092 44740490-163a-0410-bde0-09ae8108e29a