changing the bootstrap script to automatically add .precompiled.js

This (temporarily) changes the default mode of the app to testing. This will make it easier for developers - they'll always run in test mode, and won't have to worry about not committing that delta to spark.html. The build system will change the app over to none test mode before building.

We'll try and reconcile test mode, building for polymer, and developing vs. deployment in the medium term -

@dinhviethoa