Fix python-pam API usage
    
    Make authpam compatible with both the old and the new API for
    python-pam.
    Using a try when importing since that way we only need to do
    this once (upon loading the class), instead of on every request.
    
    Fixes #65
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>