Allow single component as children of ReactTransitionGroup

This is somehow related to #751 (if `@props.children` was always an array, this would work out of the box). Not sure this is desired, after all the user can wrap a single component in an array...

```coffee
# message is "some string" or null
ReactTransitionGroup transitionName: 'example',
  message
```
What I am more interested in is the ability to use inline styles instead of a CSS class (thoughts?).