Return the signed response when signing

If caller asked for a signed response, we would return None.
Return the response from self.sign() instead.
Use list addition instead of .append() for to_sign

The signing code expects a flattened list of tuples, not a list
which contains a list of tuples.  For example, in my case I had:

to_sign = [('urn:oasis:names:tc:SAML:2.0:assertion:Assertion', 'id-2d44a290a77c9fe7b50899eea96aa183'), [('urn:oasis:names:tc:SAML:2.0:protocol:Response', 'id-7416bafb9df777c1c1151b6f1ce471bb')]]

Fixes:

  File "/usr/local/lib/python2.7/dist-packages/pysaml2-1.0.2-py2.7.egg/saml2/ent
ity.py", line 458, in _response
    return self.sign(response, to_sign=to_sign)
  File "/usr/local/lib/python2.7/dist-packages/pysaml2-1.0.2-py2.7.egg/saml2/ent
ity.py", line 347, in sign
    return signed_instance_factory(msg, self.sec, to_sign)
  File "/usr/local/lib/python2.7/dist-packages/pysaml2-1.0.2-py2.7.egg/saml2/sig
ver.py", line 264, in signed_instance_factory
    for (node_name, nodeid) in elements_to_sign:
ValueError: need more than 1 value to unpack