Minimize bootstrap usage

@dinhviethoa 

This addresses #1384 to a large extent. With uncompiled code:

initPolymer takes ~1000-1300ms now vs. ~1800-2200ms before.
After the preceding step, Polymer.onReady arrives in ~100-120ms now vs. ~250ms before.
Total startup time is ~2900ms now vs ~4000ms before.

There is not much savings left related to Bootstrap usage in the widgets. If Bootstrap is completely eliminated, initPolymer takes about 700ms.