Specify specific local file location for wsdl file

Added environment variable FUELSDK_WSDL_FILE_LOCAL_LOC (and config option wsdl_file_local_loc) to allow specifying a different file location for the wsdl file. This is needed for servers that would have multiple processes running where one would connect to s4 and the other to s6. 

Also added support for 'Attributes' on TriggeredSendDefinition objects.