Try to make only sigver know about xmlsec1.
Remove obsoleted verify_signature().
Use sign_statement() instead of sign_statement_using_xmlsec().
Merge branch 'master' into less_xmlsec_paths

Conflicts:
	src/saml2/config.py
	src/saml2/httpbase.py
	tests/test_30_mdstore.py
	tests/test_31_config.py