
This method updates the key of an authority for an exisiting account.

Warning: You can create impossible authorities using this method. The
method will fail if you create an impossible owner authority, 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)
    key: The public key to add to the authority (type: public_key_type)
    weight: The weight the key should have in the authority. A weight of 0
	indicates the removal of the key. (type: weight_type)
    broadcast: true if you wish to broadcast the transaction. (type: bool)
