Workflow simplification source

This is the first of 2 coupled PRs.  This one applies only to the `source` branch. The purpose is to streamline the workflow for updating the site by building the preview in the current branch directly.  The matching PR in the `master` branch (#7) is not critical.

Testing of the final `gh-pages` capability is delicate since it, by definition, must touch the live site.

Perhaps @scopatz can review and we can propagate this workflow through to other sites/repos/projects.