Support for CSS transitions

[caged]

Fix for #18. Support CSS transitions, but in a way that doesn't require an external stylesheet dependency or a dedicated class name.

There is an example available in /examples/css-transition.html, but here are the relevant parts:

Create a selctor with your transition

  .d3-tip.animate {
    animation: bounceIn 0.2s ease-out;
    -webkit-animation: bounceIn 0.2s ease-out;
  }

Apply the selector in your event callbacks*

// snip
.on('mouseover', function(d) {
  tip.attr('class', 'd3-tip animate').show(d)
})
.on('mouseout', function(d) {
  tip.attr('class', 'd3-tip').show(d)
  tip.hide()
})

[twanlass]

Awesome work! These look great now, thanks!