Gists with since (#914)

The Github gists api has a since parameter for the various gist searches.  This adds that parameter to Github.get_gists(), NamedUser.get_gists() and Authenticator.get_gists().  Tests and replay data added/updated as needed.

I added a Time module in the tests directory, containing just a UTCtzinfo class, which I felt I needed for one of the tests (and would probably be useful in other since/until tests).  Might not be the best name for the module but it seemed the least invasive thing to do.

github/NamedUser.py

@@ -17,6 +17,7 @@
 # Copyright 2018 Wan Liuyang <tsfdye@gmail.com>                                #
 # Copyright 2018 namc <namratachaudhary@users.noreply.github.com>              #
 # Copyright 2018 sfdye <tsfdye@gmail.com>                                      #
+# Copyright 2018 itsbruce <it.is.bruce@gmail.com>                              #
 #                                                                              #
 # This file is part of PyGithub.                                               #
 # http://pygithub.readthedocs.io/                                              #
@@ -36,6 +37,8 @@
 #                                                                              #
 ################################################################################
 
+import datetime
+
 import github.GithubObject
 import github.PaginatedList
 
@@ -402,16 +405,21 @@ class NamedUser(github.GithubObject.CompletableGithubObject):
             None
         )
 
-    def get_gists(self):
+    def get_gists(self, since=github.GithubObject.NotSet):
         """
         :calls: `GET /users/:user/gists <http://developer.github.com/v3/gists>`_
+        :param since: datetime.datetime format YYYY-MM-DDTHH:MM:SSZ
         :rtype: :class:`github.PaginatedList.PaginatedList` of :class:`github.Gist.Gist`
         """
+        assert since is github.GithubObject.NotSet or isinstance(since, datetime.datetime), since
+        url_parameters = dict()
+        if since is not github.GithubObject.NotSet:
+            url_parameters["since"] = since.strftime("%Y-%m-%dT%H:%M:%SZ")
         return github.PaginatedList.PaginatedList(
             github.Gist.Gist,
             self._requester,
             self.url + "/gists",
-            None
+            url_parameters
         )
 
     def get_keys(self):