Add 'dynatable-processed' class to table when processing is complete

I needed this to avoid having the original table flash up before Dynatable had finished working on it. *e.g. with*
```css
#some-table {
   visibility: hidden;
}

#some-table.dynatable-processed {
    visibility: visible;
}
```

but debating whether it deserves to be in the core or my own event handler instead.