Build: Native Bower setup

Removes grunt-bowerful and replaces it with a native [Bower](http://bower.io) setup.
This adds bower as a global dependency (`npm install bower -g`), but we already have the same situation with `grunt-cli`. All the global dependencies are now installed as part of the postinstall hook, so this should be transparent to the use :smile: 

The other benefit of setting up (and publishing) a bower package is that the themes can then consume this package.
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Ping @jeresiv, @patheard,  @LaurentGoderre, @masterbee: could you test this tomorrow on your work machines to see if this resolves some of the issues from #2914