
This method updates the weight threshold of an authority for an account.

Warning: You can create impossible authorities using this method as well as
implicitly met authorities. The method will fail if you create an
implicitly true authority and if you create an impossible owner authoroty,
but will allow impossible active and posting authorities.

Parameters:
    account_name: The name of the account whose authority you wish to
	update (type: const string &)
    type: The authority type. e.g. owner, active, or posting (type:
	authority_type)
    threshold: The weight threshold required for the authority to be met
	(type: uint32_t)
    broadcast: true if you wish to broadcast the transaction (type: bool)
