Add make target for docset to be used with Dash/Zeal

This PR makes it simple to create a docset from the Traits html documentation, as used by the [Dash](http://kapelli.com/dash) and [Zeal](http://zealdocs.org) api browsers.

It assumes you have [doc2dash](https://pypi.python.org/pypi/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.