Add icons for elevator and u-turn instructions

The new support for using walking legs from OpenTripPlanner by default for the OpenTripPlanner transit plugin introduces a few new instruction types for turn-by-turn route parts (the walking ones), namely "elevator" (taking an elevator, getting off at some specified floor, I imagine something like the common pictogram often used at e.g. airports and rail strations, with a square with a male and female person inside and arrows pointing up and down, or something similar, and two instuctions for taking a u-turn (such as crossing a street continuing on a side-walk on the opposite side in the opposite direction. One for making the turn to the left and one to the right.