Add more helpful invariant if you're updating an unrendered component
move lifecycle check into replaceProps instead of updateComponent
Add Stack Overflow link for doc support page.
Update wording
Add ReactTransitionGroup

This introduces <ReactTransitionGroup>, a component that works a lot
like Angular's ng-animate.

The problem we're currently facing is twofold:
1. We don't really have a great convention surrounding CSS transitions
   in React
2. (harder) we can't animate nodes that are leaving the DOM, as their
   nodes are instantly destroyed.

To solve the first issue I've adopted Angular's convention. It's
implemented in ReactTransitionableChild.

The second part is what's tricky. To do this I've implemented three modules:
- ReactTransitionableChild, which can keep its old children around if they
  change to null
- ReactTransitionKeySet, which can look at a prev and next set of child
  keys and merge them in a reasonable way
- ReactTransitionGroup, which combines ReactTransitionableChild and
  ReactTransitionKeySet to keep nodes that are leaving the DOM in the
  DOM until their animations are complete
Improve error behavior of Danger.dangerouslyRenderMarkup.

HTML comment nodes are now interspersed in the original markup list so
that we can tell which chunks of markup rendered as more or less than one
node, and give a more helpful error message in that case.

Regardless of how many nodes were rendered, we only take the first one. If
zero nodes were rendered, then `resultList` will contain `null` at the
corresponding position.

If we decide to revisit the idea of using document fragments, it will be
very easy to handle the `!== 1` case by returning a document fragment.
Test cases covering rendering onto document

There were some bug reports here, I couldn't reproduce but wrote tests anyway. We should definitely have these.
Use dumpCache() rather than manual reset

I forgot this module existed until @benjamn reminded we had a way to do it.
Sync CSSCore from upstream
fix formatting and test for correct error
Merge pull request #294 from clayallsopp/better_update_msg

More helpful message if you update an unrendered component
Make mounting on the root of the page work correctly

This was apparently only partially supported. We had issues initially mounting if there was no HTML present and
also had issues if we had to update HTML that was already there. This diff fixes all of these cases and has
tests to prove it. NOTE: I removed a test that was actually erroneous. My bad.
Remove ReactChildren methods from React object

These are not terribly useful on this object and the naming of
`React.forEachChildren` sucked anyway.