[Tagging] More human readable values for traffic signs

[Mateusz Konieczny]

I recently started tagging traffic signs and I am surprised by wide
usage country-specific traffic sign codes.

I think that at least common signs may be tagged by human-readable
values. Some (see
http://wiki.openstreetmap.org/wiki/Key:traffic_sign#Human-readable_values
) are already used.

I propose to add more like
- traffic_sign=oneway
- traffic_sign=no_stopping
- traffic_sign=no_parking

In addition I propose to introduce traffic_sign_code tag that may be
used in addition to human readable values to store country-specific
traffic sign codes.