Add demos/clj-frontend and change some names

This commit Mainly adds demos/clj-frontend based on examples prepared for the [wiki page](https://github.com/Factual/drake/wiki/A-Clojure-Frontend-to-Drake) for clj-frontend.

In addition to adding a demo, this commit also modifies clj-frontend by
changing the `workflow` function to `new-workflow` and changing 
`parse-tree` to `w-flow` in the inputs to several functions.  Changing 
parse-tree to w-flow seems to better hide the workflow implementation 
details from users.