Feature/saml metadata extension mdattr

Adds support for EntityAttributes per http://docs.oasis-open.org/security/saml/Post2.0/sstc-metadata-attr-cs-01.pdf

Note that I've explicitly not added support for 2.4 as we are aiming to utilise http://macedir.org/draft-macedir-entity-category-00.html only at this time. Support for the Assertion detail doesn't seem to be a real need and would be a reasonable overhead of models/code. 
Given that we're only using Entity Category at this stage, is there any point to having this association? My understanding is it only needs to exist on the `EntityDescriptor` model.