Avatar Size Slider + Hero Names

Here's another one for you. This pull request adds an avatar size slider for the Available Heroes section, and displays hero names (overlaid on avatar) when at max size. Intended for newbies who might have a harder time recognizing all the heroes (or knowing all their names). Uses the HTML5 slider element, with a polyfill (http://andreruffert.github.io/rangeslider.js/) for older browsers (plus it looks a little nicer). Tested in Chrome, Firefox v29, IE 11, and Android (Chrome).