fix 404 errors on bower_components with Gulp on Windows

[aledelgo]

fix(app): fix 404 errors on bower_components with Gulp on Windows

With Gulp, the serve command don't expose symlink (or virtual dir) to bower_components on windows, so http://localhost:9000/bower_components/any_component/aa.js return a 404 error

This fix make "bower_component" a virutal dir for static file via "st" module as middleware for "connect" module. Now it works.

I have no test actually.