This is designed to let users rate how happy they are with something using a 
smiley faced slider.

Use:

    // add the slider to a div with id "slider"
    var s = new SmileySlider(document.getElementById("slider"))
    
    s.position(0) // make it sad
    s.position(1) // make it happy
    
    var p = s.position() // get it's position
    s.position(p / 2) // make it half as happy
    
    s.position(function (p) {
        // do something when it changes
    })