Add dynamic placement option for tooltip

I have added an option to the tooltip plugin, for dynamic positioning. I was looking for this, and I noticed some other people thought that having this would be beneficial as well.

```javascript
$('.tip').tooltip({
    dynamicPos: true
});
$('.pop').popover({
    dynamicPos: true
});
```

I have created a fiddle to show its usage: http://jsfiddle.net/nickroberts/U4384/3/