Move redirect and post to class SLO.

Since SSO overrides redirect and post from Service, the code
in Service was actually only used in SLO - so moved there for
clarity.
Remove unused 'uri'.

Left over from pysaml2 example IdP.
SOAP only supported for SLO, so move code there.
Artifact only supported for SSO, so move code there.
Fix imports.
Refactor away non-descript functions operation() and do().
Tinkering.

Mostly PEP-8 and style changes due to testing PyCharm.
Refactor