logout request support

This PR adds initial support for SAML Logout Requests in order to implement SLO.

More specifically:

- logout request support in bindings
- create logout request objects from xml
- support for format, service provider id and service provider
  qualifier attributes when generating xml in SAML name identifiers
  [rsaml/identifier/name.rb]

Also I needed to add `active_support/core_ext` to make tests green again.

Cheers.