cpp: Add doxygen API reference

Testing:

- Needs doxygen and graphviz to be installed
- Run cmake as normal
- Run make to do a complete build (to generate all sources)
- Run "make doc"; no errors should be logged (you'll get a warning if so, and it will also be in doc/bioformats.log)
- Built documentation is under doc/bioformats; you should have a complete API reference for all the available C++ classes and methods.

See also:

- http://hudson.openmicroscopy.org.uk/view/All/job/BIOFORMATS-merge-develop-cpp/doxygen/annotated.html
- http://hudson.openmicroscopy.org.uk/view/All/job/BIOFORMATS-merge-develop-cpp/lastSuccessfulBuild/artifact/cpp/build/install/bioformats-cpp-apidoc-5.0.0-beta1-462-ga429549.zip (the git hash will change)