Changed the name of 'DEFAULT_FORMAT' to 'DEFAULT_DATA_FORMAT' as it is more explicit. Altered SteamWebAPI.format to SteamWebAPI.data_format due to format() being a Python built-in function. Added logic to the SteamWebAPI.__init__() to check that the 'key' argument give is valid (see API_KEY_RE). Also added logic to insure SteamWebAPI.data_format is valid.