Callback beforeOpen to execute code before showing menu (with documentation)

This adds a callback which can be provided with:

```
$("#whatever").contextPopup({
    beforeOpen : function(e) {
    // ..
});
```

This is very useful in situation where you want to change something before actually showing the menu. For example I use it to select a table row (i.e. paint it orange) before showing the menu for it.

Documentation is included in the header and in the README.