Add a way to download stickers (fixes #36)

This is marked as a bugfix in the changelog because this was a
documented feature for botogram 0.1, and its absence is a bug.

Author: Pietro Albini

botogram/objects/__init__.py

@@ -221,7 +221,7 @@ class Document(BaseObject, mixins.FileMixin):
     }
 
 
-class Sticker(BaseObject):
+class Sticker(BaseObject, mixins.FileMixin):
     """Telegram API representation of a sticker
 
     https://core.telegram.org/bots/api#sticker

docs/changelog.rst

@@ -43,6 +43,7 @@ Bug fixes
 
 * Fix the syntax detector checking URLs with dashes in the domain (`issue 32`_)
 * Fix inability to send messages to channels from a running bot (`issue 35`_)
+* Fix inability to download stickers (`issue 36`_)
 
 Deprecated features
 -------------------
@@ -54,6 +55,7 @@ Deprecated features will be removed in botogram 1.0!
 
 .. _issue 32: https://github.com/pietroalbini/botogram/issues/32
 .. _issue 35: https://github.com/pietroalbini/botogram/issues/35
+.. _issue 36: https://github.com/pietroalbini/botogram/issues/36
 
 .. _changelog-0.1.2: