Settle on NESW or UDLR for all parts of interface.

[wbthomason]

Different parts of the interface specify directions as NESW or as UDLR. In particular, to unify on UDLR, we would need to change the Picobot parser. It is also not clear which of these is the better convention. UDLR is arguably clearer, but becomes confusing when doing the maze exercise (because "follow the left wall" implies that the Picobot is turning, which changes what UDLR means for its reference frame). In general, we should just pick one convention to use for all parts of the interface.