Add support for the MDUI extension.

This patch adds support parsing and generating metadata with the
mdui:UIInfo and mdui:DiscoHints elements.

Support for generating metadata with the extensions is added to the
SAML 2.0 IdP. It should also work through the metadata aggregator.

Thanks to Timothy Ace at Synacor, Inc. for implementing this!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3088 44740490-163a-0410-bde0-09ae8108e29a