Consolidate rdf2hdt Windows-specific implementation and base implementation

[donpellegrino]

The develop branch currently has both /rdf2hdt/rdfhdt.cpp and /libhdt/tools/rdf2hdt.cpp implementations of the rdf2hdt command-line interface. The Git history indicates that the /rdf2hdt/ directory was created to accommodate Microsoft Windows. It may be easier to maintain a single implementation of the rdf2hdt CLI in the /libhdt/tools directory along with the other tools and let the build system sort out platform differences.