UI for audio mute, video mute, camera switch, fullscreen and hangup

Uses inline SVGs to avoid slowing down page load. (These are [well supported](http://caniuse.com/#search=inline%20svg).)

Also added:
• `$()` function for `document.querySelector()`
• `hide()` and `show()` functions to add/remove the class name `hidden`
• `activate()` and `deactivate()` to do the same for `active`

Getting this error with jstdPhantom Grunt task, not sure why: 

> Safari: Reset
> Safari: Reset
> ...
> Total 3 tests (Passed: 3; Fails: 0; Errors: 0) (4.00 ms)
>   Safari 534.34 Mac OS: Run 4 tests (Passed: 3; Fails: 0; Errors 1) (4.00 ms)
>     error loading file: /test/js/main.js:60: TypeError: 'null' is not an object (evaluating 'muteAudioSvg.onclick = toggleAudioMute')
> 
> &gt;&gt; Total Passed: 3, Fails: 0
> 
> PhantomJS threw an error:
> Done, without errors.
> 
Name, and title should probably be changed to "AppRTC" now. description can continue to be "WebRTC reference app"