Switch urllib2 to requests

This patch switches the http library used by pykol from urllib2 to requests.  The latter library is generally more capable, has a friendlier API, and importantly, is able to avoid a new HTTP request every time a resource is requested from the server.

This passes the unit tests.  Changes were minimal; the key changes were to Session.py and GenericRequest.py, and the only other changes necessary were to switch response.getUrl() to response.url.  (Perhaps that should be abstracted?)