Add parse and format hooks

The idea here is you can do something like:

            @$input.knob {
                thickness: ".2",
                readOnly: true,
                width: "60%",
                height: "60%",
                angleArc: 250,
                angleOffset: -125,
                parse: function (v) {return parseInt(v);},
                format: function (v) {return v + "%";}
            }

The parse and format handlers will strip and add the "%" sign, respectively, so you get a knob that looks like:

![screen shot 2013-11-01 at 10 56 04 am](https://f.cloud.github.com/assets/1771003/1454262/c5671810-4305-11e3-8c7d-f37ce0609b99.png)

It will show the "%" even if you drag with the mouse or use arrow keys or what have you.  You could use this to add a "$" or whatever unit you want, or even to show values in hex.