properly handle timezones and a few small unittest fixes

Hi all,

I see in onelogin.saml.Response strptime being used on line 61. This causes a problem when receiving a datetime string with a timezone specification. It is possible to use dateutil.parser.parse instead, which parses a datetime string better, and returns a datetime.datetime object with the tzinfo attribute set. 
It does require python-dateutil which can be installed using easy_install. 
I added this functionality, including unittests, and modification of setup.py and README.rst.

Aside from this, I fixed two unittests which failed before these changes already (AuthnRequest object was aparently renamed, but unittests were not, and my previous fix on NotBefore not being required was not reflected in unittests).

Cheers,

Dolf Andringa.