Add support for logout over SOAP
    
    As each login session comes in, store the supported logout
    mechanisms in the SP metadata.
    
    Upon a logout request, loop through all of those SP's that
    support SOAP and log those out first, then log out any
    remaining sessions using HTTP Redirect.
    
    https://fedorahosted.org/ipsilon/ticket/59
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>