Feature/saml metadata extension mdrpi

Implements models to support SAML V2.0 Metadata Extensions for Registration and Publication Information Version 1.0
per http://docs.oasis-open.org/security/saml/Post2.0/saml-metadata-rpi/v1.0/saml-metadata-rpi-v1.0.pdf

Models for section 2.3 are not provided as this data would be dynamically created from 
supplied data during metadata generation time.