ENH: add HTML Help builder option

[cgohlke]

This patch allows to build documentation in HTML Help format. See also discussion on matplotlib-devel.
On Windows, some symlinks need to be created manually before running python make.py --small htmlhelp:

del mpl_examples
del mpl_toolkits\axes_grid\examples
mklink /D mpl_examples  ..\examples\
mklink /D mpl_toolkits\axes_grid\examples ..\..\..\examples\axes_grid\

The CHM file, Matplotlibdoc.chm, can then be generated with hhc.exe Matplotlibdoc.hhp.
The HTML Help compiler, hhc.exe, is part of the Microsoft HTML Help Workshop 1.3.