Added afterShowCallback

I had some problems with corner detection and floated elements then I came up with a solution of adding `offsetX` and `offsetY` data attributes to `<li>` and then adjusting each slide position inside a callback function.

Then I noticed there is no callback to be called right after the tip is showed.
This patch, adds an simple `afterShowCallback`.

My callback, finally, was like this coffeescript piece of code:

```coffeescript
'afterShowCallback': () ->
  x  = this.$current_tip.offset().left
  y  = this.$current_tip.offset().top
  dx = this.$li.attr('data-offsetX')
  dy = this.$li.attr('data-offsetY')
  dx = if dx == 'undefined' then 0 else parseInt dx
  dy = if dy == 'undefined' then 0 else parseInt dy
  this.$current_tip.offset({left: x+dx, top: y+dy})
```