Fix scrollLeft/scrollTop warning in latest Chrome

Chrome gives the warnings

```
body.scrollLeft is deprecated in strict mode. Please use 'documentElement.scrollLeft' if in strict mode and 'body.scrollLeft' only if in quirks mode.
body.scrollTop is deprecated in strict mode. Please use 'documentElement.scrollTop' if in strict mode and 'body.scrollTop' only if in quirks mode.
```

the first time getUnboundedScrollPosition is invoked. This fixes it. (All modern browsers support `pageYOffset`; IE 8 doesn't but works properly with `document.documentElement.scrollTop` except in quirks mode which React doesn't support.)
Community Round-up #14
Make controlled components and bubbling work in IE

Fixes #708.

Test Plan:
In IE9, tested a controlled text input with the event handler on a containing element, as in the fiddle linked in the original issue. Also tested a controlled radio button as the logic there differs within ReactDOMInput. In both cases, I was able to interact with the controls.
Don't return null children from flattenChildren

This simplifies ReactMultiChild a little bit and will make it more practical to coalesce adjacent text strings.
Add onMouseOver and onMouseOut events

Fixes #340.

Test Plan:
Ported @danielstocks's jsfiddle (linked in the issue) to React and the hover effect worked properly.
Add source map support to browser transformer
Remove sometimes-confusing console error

Fixes #767. This essentially reverts 738de8c.

We could store some sort of flag to silence the console error here but since we've made significant improvements in markup wrapping, this error is fairly rare now. We'll also have validation of node structure soon in #735.
Set target properties explicitly for enter/leave

Fixes #785.
Don't add displayName in transform if already defined

This ensures we don't add displayName in the transform if it's already been
defined. This is especially important when in strict mode, where duplicate
properties in an object is an exception.
Merge pull request #721 from spicyj/change-bubble-ie

Make controlled components and bubbling work in IE
Remove mentions of browserify in examples
Mention react-art where we talk about SVG
Merge pull request #805 from spicyj/docs-mention-react-art

Mention react-art where we talk about SVG
Fix typo
Merge pull request #806 from spicyj/patch-4

Fix typo
Prefer textContent to innerText

Fixes #807.
Listen to events on demand

Fixes #381

This is a squashed version of https://github.com/facebook/react/pull/462
Set currentTarget on synthetic events

Fixes #658, fixes #659.
Add gulp-react to Helpful OSS Projects

Add @sindresorhus' JSX task for Gulp to the "Helpful Open-Source Projects" section of the tooling integration docs.
Document displayName