Added afterShowCallback

[acasimiro]

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:

$('#joyride').joyride {
  'afterShowCallback': () ->
    x  = this.$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})
}