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
Fix npm react having wrong version dependency for envify
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.
Add invariant to check getInitialState return value. Fixes #397
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.
Don't get selection if no active element

Fixes #694.

Previously, it could be that there was an active element that was removed in between mousedown and mouseup.
docs tips fix small typo and code
Merge pull request #775 from chenglou/doc-fix

docs tips fix small typo and code
docs better error display for live editor and JSX compiler
Merge pull request #779 from chenglou/live-err

docs better error display for live editor and JSX compiler
Merge pull request #725 from cpojer/getInitialState-invariant

Add invariant to check getInitialState return value. Fixes #397.
Upgrade commoner build tool to v0.8.12.

Fixes #394.
Fixes #615.
React Chrome Developer Tools
Merge pull request #784 from vjeux/react_dev_tools

React Chrome Developer Tools
Merge pull request #783 from benjamn/upgrade-commoner

Upgrade commoner build tool to v0.8.12.
Set target properties explicitly for enter/leave

Fixes #785.