Add support for sending responses with the HTTP-Artifact binding.

This patch implements support for sending responses to authentication
requests via the HTTP-Artifact binding. To enable, add
'saml20.sendartifact' => TRUE in saml20-idp-hosted metadata. The IdP
should then send HTTP-Artifact responses to SPs that request it.

Note that this requires a working memcache server.

Thanks to Danny Bollaert for implementing support for this.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2121 44740490-163a-0410-bde0-09ae8108e29a