Support for CSS transitions

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](examples/css-transition.html), but here are the relevant parts:

**Create a selctor with your transition**

``` css
  .d3-tip.animate {
    animation: bounceIn 0.2s ease-out;
    -webkit-animation: bounceIn 0.2s ease-out;
  }
```

** Apply the selector in your event callbacks***

``` javascript
// 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()
})
```