Add bower dependencies option

In thinking more about a `frontEndDependencies` option from #9 I figured the more standard strategy would be to use Bower. The option solves the problem where a client-side dep may be depending on another library that hasn't been loaded yet. And gives us the same benefits of selective dep loading that `npmDependencies` gives us.

The code assumes your bower installation directory is `bower_components`. A slightly more robust implementation might check if a .bowerrc file exists with the directory option set.