Adding new attributes to IssueEvent (#857)

See Issue #855 

The class [IssueEvent](https://github.com/PyGithub/PyGithub/blob/master/github/IssueEvent.py) is missing a large number of attributes documented in the [API](https://developer.github.com/v3/issues/events/).

This is also commented about in #653 to a degree

27 of the tests 27 known event types have tests.

**Currently Tested using Issue #30**            
- [x] subscribed
- [x] assigned
- [x] referenced               
- [x] closed                   
- [x] labeled                  

**Currently Tested using Issue/PR #538**
- [x] merged
- [x] mentioned
- [x] review_requested

**Currently Tested using Issue/PR #857**
- [x] reopened
- [x] unassigned
- [x] unlabeled
- [x] renamed
- [x] base_ref_changed
- [x] head_ref_deleted 
- [x] head_ref_restored
- [x] milestoned
- [x] demilestoned
- [x] locked
- [x] unlocked
- [x] review_dismissed
- [x] review_request_removed
- [x] marked_as_duplicate
- [x] unmarked_as_duplicate
- [x] added_to_project       
- [x] moved_columns_in_project
- [x] removed_from_project
- [x] converted_note_to_issue - Note: this event is tied into Issue #866 

This PR is now ready to be merged

github/Repository.py

@@ -1925,7 +1925,8 @@ class Repository(github.GithubObject.CompletableGithubObject):
         assert isinstance(id, (int, long)), id
         headers, data = self._requester.requestJsonAndCheck(
             "GET",
-            self.url + "/issues/events/" + str(id)
+            self.url + "/issues/events/" + str(id),
+            headers={'Accept': Consts.mediaTypeLockReasonPreview}
         )
         return github.IssueEvent.IssueEvent(self._requester, headers, data, completed=True)
 
@@ -1938,7 +1939,8 @@ class Repository(github.GithubObject.CompletableGithubObject):
             github.IssueEvent.IssueEvent,
             self._requester,
             self.url + "/issues/events",
-            None
+            None,
+            headers={'Accept': Consts.mediaTypeLockReasonPreview}
         )
 
     def get_key(self, id):