Add _media methods and support for resumable media download.

TBR: http://codereview.appspot.com/6295077/
diff --git a/apiclient/model.py b/apiclient/model.py
index aafd88b..0e31053 100644
--- a/apiclient/model.py
+++ b/apiclient/model.py
@@ -289,6 +289,25 @@
     return ''
 
 
+class MediaModel(JsonModel):
+  """Model class for requests that return Media.
+
+  Serializes and de-serializes between JSON and the Python
+  object representation of HTTP request, and returns the raw bytes
+  of the response body.
+  """
+  accept = '*/*'
+  content_type = 'application/json'
+  alt_param = 'media'
+
+  def deserialize(self, content):
+    return content
+
+  @property
+  def no_content_response(self):
+    return ''
+
+
 class ProtocolBufferModel(BaseModel):
   """Model class for protocol buffers.