Add compilation with closure compiler to travis for all elements

[rictic]

This is desirable as it will catch a number of bugs and documentation errors, as well as reducing the impedance mismatch for folks who want to compile Polymer.

Steps:

• prototype a basic open source closure compiler flow
• test out travis integration
• apply the travis integration for all elements with tedium

[GeoloeG-IsT]

Thx! /sub