add a bower servlet to the launch server

@ussuri

Add a Bower servlet to the launch server. This servlet can resolve `../` references to Bower packages - i.e., it can work with a situation where the bower package assumes that all the components are siblings on disk. It serves a request for `/polymer/polymer.js` from `/FooProject/bower_components/polymer/polymer.js`.

This depends on us knowing what the current project is. We do this by recording the last launched project. This will work in most cases by is a bit fragile. In the future we should change to a mode which creates a server (on a separate port) for each project.

These components can now be used by Spark: https://github.com/GoogleWebComponents. @ussuri, I had to edit the bower.json file to remove explicit references to git branches.