ENH: add HTML Help builder option

This patch allows to build documentation in [HTML Help format](http://en.wikipedia.org/wiki/Microsoft_Compiled_HTML_Help). See also discussion on [matplotlib-devel](http://matplotlib.1069221.n5.nabble.com/Please-consider-Html-Help-td43692.html).
On Windows, some symlinks need to be created manually before running `python make.py 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`.