Fix virtual methods in doxygen2boostbook.
Add newlines to doxygen2boostbook output.

Hopefully this won't break anything, and will make it much easier to read the
intermediate files.
Remove unused date handling code from html-help.
Try to make last-revsion a little more robust.

Don't display anything for empty '$Date$' variables, and just display the value
for normal, non version control values.
Factor out method and function attributes.

Not all attributes apply to all methods and functions, but hopefully they should
never appear when inappropriate.
Avoid confusing arguments with quialifiers from argsstring.
Less verbose handling of qualifiers.
Add noexcept and constexpr to doxygen to boostbook coversion.

Should really add proper support to boostbook for `=delete`, `=default` and
noexcept, but I'm not feeling that keen right now. I'm not sure I did the right
thing with `BOOST_CONSTEXPR_OR_CONST`.
Fix support for const, volatile, and pure virtual.
Use specifiers attribute for virtual and explicit.