Get list of all volumes or info for the one specified.

def volume_info(self, vol=None, type_filter=None):
		'Get list of all volumes or info for the one specified.'
		vols = yield self(join('volumes', vol))
		if not isinstance(vols, list): vols = [vols]
		if type_filter is not None:
			vols = list(vol for vol in vols if vol['type'] == type_filter)
		if vol is not None: defer.returnValue(vols[0] if vols else None)
		defer.returnValue(vols)