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

[MeTaNoV]

Thx! /sub