Documentation build modifications:

- add macro "_add_doxy_headers" in order to track all header files elligible for
  doxygen documenation. This captures public header files that would otherwise be
  excluded from installation because they are not supported by the OS. Private
  header files remain excluded though.

- add custom targets and commands so that documentation build produces functioning
  RST and Doxygen documentation both in the build and install stages

- switched to Doxygen 1.8 (because markdown will make in-lined documentation easier)

- added build switches to disable examples, regression and python-SWIG targets

- fixed doxygen link in the nav bar

- modified python html processing tool to match Cmake changes