Added host setting to GSBotoStorage

The value of this setting used to be derived from the parent class, S3BotoStorage. The value in the parent class was: `host = setting('AWS_S3_HOST', S3Connection.DefaultHost)`, which resolved to 's3.amazonaws.com' instead of the correct 'storage.googleapis.com'.
This fixed jschneier#124 for me.

storages/backends/gs.py

@@ -63,6 +63,7 @@ class GSBotoStorage(S3BotoStorage):
         'application/x-javascript',
     ))
     url_protocol = setting('GS_URL_PROTOCOL', 'http:')
+    host = setting('GS_HOST', GSConnection.DefaultHost)
 
     def _save_content(self, key, content, headers):
         # only pass backwards incompatible arguments if they vary from the default