Parse multiple docs

[mojodna]

This seeds the XML parser so that it can happily handle multiple docs (or multiple fully closed inputs), e.g. repeated <OsmChange /> fragments.

[gmaclennan]

Looks good @Mojoda seems a little hacky, but doesn't seem to break any tests.
Looking at this module reminds me that I should rename it at some point to osm2obj.

[mojodna]

Agreed, but the logic seems sound (end of doc == end of stream; don't end the doc, don't end the stream). Idea originally from isaacs/sax-js#205