osm2shp: Use Osmium as input handler instead of our own XML/expat implementation

This enables us to read also PBF files
Merge pull request #1 from Turbo87/master

Build fixes
Makefile: Added proper CXXFLAGS for parsing large files

Parsing the 14 GB planet.osm.pbf fails without these flags.
Merge pull request #2 from Turbo87/osmium

Osmium input reader