add a glow effekt

get a glow effekt with the follow example
```js
$(".knob").knob({
 	"glow" :30,                                 // 30 is a good example
 	"glow_text": 4,                           // 4 is a good example
 	"glowcolor":"cyan",

       "fgColor" : "cyan",
       "displayPrevious":false, 
       "lineCap": "round",
       "bgColor": "transparent",
       "thickness": 0.15,
 });
```
![glow](https://f.cloud.github.com/assets/4940490/1289411/09b2d132-3021-11e3-849f-a0d885dae58e.jpg)