Support showing exit signs of subways

When traveling with a subway, it is very helpful to know which exit should be taken to leave the station. TripKit should parse this info and include it in its response. The information could potentially be stored in the IndividualLeg class.

Efa contains an XML element that could be parsed easily: <guidanceSignElem signType="DIRECTION" text="Ausgang F" url=""/>

GTFS name is signposted_as