add wrapper option

it would be useful adding a `wrapper` or `parent` option so you can always set the parent where to add the tooltip, instead of fixing it to the `body`. This is an example of how that code would look like:

```
var $wrapper = options.wrapper || $('body')

$wrapper.append('<div class = "margintooltip" style = "display:none;"></div>')
```

Congrats for the snippet, and spurring such inspiration.
