Bit of more house/build work:
 * Added ability to build Doxygen-based documentation.
 * Improved pplme::core::DiscriminatedValue example (wasn't using a namespace
   so ADL "benefits" weren't being illustrated that effectively).
 * Makefile best practices.
 * Beefed up DiscriminatedValue.
 * Followed what seems to be common naming practices for GTest test fixture
   names.
 * Introduction of first blush at pplme::core::GeoPosition.
IT IS DONE.  THE DOMAIN.  ALL HAIL THE DOMAIN.