Update ClojureScript, lein-cljsbuild, Om, React.
Add :preamble to advanced profile to enable compilation.

According to the Om documentation, it's necessary to define both
`:preamble ["react/react.min.js"]` and `:externs
["react/externs/react.js"]`. In practice, I found that adding the
`:externs` declaration caused a "duplicate extern" error from the
Closure compiler. Maybe the React file is self-externing? It comes from
a jar pulled in as an Om dependency, so maybe it has some extra niceties
specifically for ClojureScript.
Temporarily hack index.html to use advanced-compile JS file.

After advanced compilation, there is a single JS file which contains
everything but fastclick.js. We can just include it and let it work.

In the dev build, however, we have separate script tags for goog/base
and our own code, and then we call `goog.require` on our core namespace
to run the app. Better for debugging.

For now, we'll just comment out the version we don't want. Soon we want
build-specific HTML files, so we don't have to keep changing this one.