Exit on argument errors in stlpd-download.
New tool: bin/osm-slice

- Add the bin/osm-slice tool to slice apart a (large) OSM file into only
  the portion necessary for representing a given relation. This is
  relatively slow since it needs to make several passes over the OSM
  file to avoid pulling it all into memory.
- Simple osm.js library to aid in parsing and working with OSM objects.
Add OSM relation viewer at /_/osm_view

- Add /_/osm_view, which supports GET and POST. The former renders a
  form for populating the latter.
Add XXX comment to index.html about GA
Move some static files around
Add explanatory text to OSM viewer form
Remove superfluous argument from osm.waysToContiguousNIDs.
Add sendFailureResponse() helper to index.js
Add osm.readFullStream(); use in region viewer
Add regions/ API for returning GeoJSON of supported regions.