SAML2: Support configurable attribute encodings.

This option makes it possible to include raw XML in the attributes
sent to an SP. A new option is introduced: attributeencodings

Patch by Joakim Recht <jre@trifork.com>.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1459 44740490-163a-0410-bde0-09ae8108e29a