Split Game object to separate Activity subtypes for Rich Presences.

This is a massive breaking change.

* All references to "game" have been renamed to "activity"
* Activity objects contain a majority of the rich presence information
* Game and Streaming are subtypes for memory optimisation purposes for
  the more common cases.
* Introduce a more specialised read-only type, Spotify, for the
  official Spotify integration to make it easier to use.

discord/enums.py

@@ -28,7 +28,8 @@ from enum import Enum, IntEnum
 
 __all__ = ['ChannelType', 'MessageType', 'VoiceRegion', 'VerificationLevel',
            'ContentFilter', 'Status', 'DefaultAvatar', 'RelationshipType',
-           'AuditLogAction', 'AuditLogActionCategory', 'UserFlags', ]
+           'AuditLogAction', 'AuditLogActionCategory', 'UserFlags',
+           'ActivityType', ]
 
 class ChannelType(Enum):
     text     = 0
@@ -212,6 +213,14 @@ class UserFlags(Enum):
     partner = 2
     hypesquad = 4
 
+class ActivityType(IntEnum):
+    unknown = -1
+    playing = 0
+    streaming = 1
+    listening = 2
+    watching = 3
+
+
 def try_enum(cls, val):
     """A function that tries to turn the value into enum ``cls``.