folder/rename - Renames an existing folder.

Method parameters:
id - The ID of the folder to rename.
name - The new name of the folder.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
FOLDER_NOT_FOUND - A folder with the ID doesn't exist.
ENTER_NAME - The new name is empty.
FOLDER_ALREADY_EXISTS - A folder with the name already exists.