New onboarding pages, polymer paper elements

* New onboarding pages for uProxy
 * Initial splash page
 * Description page
 * Get / Share choice (buttons currently just advance to next screen, will take effect once we re-implement modality tabs)
 * New network login page (moved from networks.html to splash.html)
* Polymer paper (material design) elements
 * New grunt task to compile paper-*.html elements by moving their inline scripts to a separate .js file (inline scripts are not allowed in Chrome extensions)

Tested in UI (chrome popup), re-ran "grunt test"
Our UI doesn't support zoom and all fonts throughout uProxy are currently in px.  We'll re-evaluate this later when we implement Roboto (https://github.com/uProxy/uproxy/issues/577)