Use lein-bower to download JavaScript and CSS assets

Just reject this PR if you were headed in a different direction with Bower.

I initially struggled with building a JAR that included the CSS and JS assets.  First used:

    npm install bower
    (cd resources/public; ../../node_modules/bower/bin/bower install)

but the lein plugin approach seems more aligned with the rest of the project.

If this is accepted, the next step might be to better integrate the bower steps with the rest of the
build so one could just clone the repo and `lein uberjar` to get a fully populated standalone JAR.