Id attributes changed to customizable class names

I changed all ids to classes so the plugin can be called twice or more in a web page. Also I added options for anyone wants to change classes

``` javascript
    $selector.pivotUI({
        rowsClass: "rows",
        colsClass: "cols",
        valsClass: "vals",
        unusedClass: "unused",
        aggregatorClass: "aggregator",
        rendererClass: "renderer"
    })
```