Added a data-limit option to the plugin.

**Use**
Add a data-limit with an integer value to the <select> object, like so:
```html
    <select id="mylist" multiple="multiple" data-limit="3">
```

Optionally, send in a limit_reached function to handle a selection beyond the limit that has been set.

This is in answer to a StackOverflow problem: http://bit.ly/16PINiw

Example jsFiddle on use: http://jsfiddle.net/mori57/L5EMy/