make conversion of osm-json slightly faster (~25%++)
faster geojson output of large datasets

JSON.stringify() is really slow on large datasets and this step also
delays output response significantly.

A naive benchmark shows an improvement from 4m40s to 41s (for a
160MB osm-json input file).
faster xml parsing

using a streaming xml parser (htmlparser2) instead of xmldom speeds
up things significantly. especially with very large xml files.
fix bug in json output construction