Create PostAndCommentService, holds common methods

Some functionality such as saving, voting, deleting, etc. is common to
both posts and comments, so this service will hold such methods.

Tweaked the separate post/comment services accordingly. Fixed tests.

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

geddit_test.go

@@ -70,11 +70,13 @@ func testClientServices(t *testing.T, c *Client) {
 		"Comment",
 		"Flair",
 		"Listings",
+		"Moderation",
+		"Multi",
 		"Post",
+		"PostAndComment",
 		"Search",
 		"Subreddit",
 		"User",
-		"Vote",
 	}
 
 	cp := reflect.ValueOf(c)