Fix README and add missing dep

When I tried to run master with the given instructions, I ran into two snags: one was that the JS url to test was om-whitespace-js.stefon, which wasn't there because I ran "cljsbuild auto dev" from the instructions above.  The other was that cljsbuild was complaining that it couldn't find the namespace cemerick.url.  I included the latest version in project.clj, which seems to work on my machine.

* Adds cemerick/url to project deps to satisfy github.cljs
* Updates README to point to test om-dev.js instead of om-whitespace.js