All HTML code and Doxygen shortcuts such as @c, @b and @em are now
rewritten using Markdown syntax, which makes it more readable.
Also updated Doxyfile. Doxygen 1.8 at least is required to generate
the documentation now.
SubdivideCleanBenchmark times are untouched with this change, but the
hash should be better for MeshTools::CombineIndices, where the previous
hash generated all zeros in most cases.