Add Dash/Zeal docset make target

Just like enthought/traits#180, this PR introduces a make target for a Dash/Zeal type docset.

It assumes you have doc2dash installed and keeps all output under $(BUILDDIR).

The docset_html target lacks the left sidebar to make viewing in Dash/Zeal nicer. Unfortunately, sphinx does not support the notion of configuration files named anything other than conf.py and so we must make a directory docset_resources in which to hold the modified configuration. If there is a way to tell sphinx-build to override the html_theme_options dictionary then this directory is unneeded.
This will evaluate as True whether `BUILD_DOCSET` is set to `0` or `1` in the environment.