Added host setting to GSBotoStorage (fixes #124)

[dcgoss]

GSBotoStorage inherits this method from its parent class, S3BotoStorage:

def connection(self):
        if self._connection is None:
            self._connection = self.connection_class(
                self.access_key,
                self.secret_key,
                is_secure=self.use_ssl,
                calling_format=self.calling_format,
                host=self.host,
                port=self.port,
                proxy=self.proxy,
                proxy_port=self.proxy_port
            )
        return self._connection

Note the host=self.host.

The value of this setting for GSBotoStorage used to be derived from S3BotoStorage as well. 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 #124 for me.

[dcgoss]

This should also resolve #54, which is the same issue.