Fix hover-popover/tooltip flickering when mouse re-enters

I'm not exactly happy with the code change I've made to fix this issue, but this at least demonstrates a fix (as well as the bug itself).

JSBin:  http://jsbin.com/hajeta/2/

Issues:
- Mouse out / mouse in over elements within the popover, the popover will continually flicker (until your mouse hovers over *only* the popover itself.  no child elements)
- Mouse out / mouse in, if the user scrolled within the popover -- that scorll position is lost (b/c the popover is recreated repeatedly)

Steps to trigger the flickering bug:
- Hover over the text "Hover here!"
- Mouse over the tooltip itself
- Mouse-out and then quickly mouse back in (specifically, mouse back in over the text itself)
- See a flicker.  Then mouse up over other text.  The flickering continues.

Additionally: You can scroll within the popover and see the flicker / (continual) resetting of the inner scrollbar position.

Hopefully the explanation and the demo make sense.

Thank you very much!  Bootstrap is fantastic.

Change to:
```js
var tipId = isVisible ? $tip.attr('id') : this.getUID(this.type)
```