Add hooks to retrieve last-sent and last-received requests and responses

[pitbulk]

To get more data for troubleshooting in production, we've added hooks to allow us to pull the relevant XML documents out of python-saml.

With the methods:

• get_last_request_xml
• get_last_response_xml

It able to get:

• Last SAMLResponse processed by process_response
• Last SAMLRequest sent by login
• Last SAMLRequest sent by logout
• Last SAMLRequest processed by process_slo
• Last SAMLResponse processed by process_slo
• Last SAMLRequest sent by process_slo
• Last SAMLResponse sent by process_slo

[haikuginger]

Looks good to me @pitbulk! Thanks so much!