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
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.
Set target properties explicitly for enter/leave

Fixes #785.
Further cleanup of isEventSupported
Fix WheelEvent incorrectly inverting sign of deltaY
Remove mentions of browserify in examples
Prefer textContent to innerText

Fixes #807.
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.
Add link to HTML to JSX converter to “JSX In Depth” page
Change “not allowed” back to “discouraged” since you technically *can* use props called `class` or `for`…
Added support for 4 extra SVG tags: stop, defs, linearGradient, radialGradient
Updated docs with new SVG elements
Merge pull request #818 from Daniel15/link-htmltojsx

Add link to HTML to JSX converter to “JSX In Depth” page