Expose some more addons

This adds some more addons.

If this is accepted we will update the documentation to have instructions for using addons from `npm` as well as from the premade builds. If using `npm` you should `require('react/lib/ReactTransitionGroup')` rather than using `react/addons` so you don't pull in extra dependencies.

I think we definitely need the event plugins and TestUtils. I wasn't sure about `ImmutableObject` and `StateSetters`, but they seem like good ideas. I considered adding `ReactUpdates.injection` in there for pluggable batching strategies, but this already works in npm today (`react-raf-batching`) and I wanted to keep this as constrained as possible.

If someone signs off on this I'll stack a commit on top with docs (or do it in a new PR, either way).