Add debug logging of lasso library

If debug is enabled make lasso spit debug messages to stderr too, to aid
admins in resolving issues related to saml2 issues, like finding out why
a metadata file may be rejected.

This is very simple for now, a future enhancement may involve piping the
logs into a calss so they can be spat out as feedback to users.

Signed-off-by: Simo Sorce <simo@redhat.com>