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.)
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`…
Merge pull request #818 from Daniel15/link-htmltojsx

Add link to HTML to JSX converter to “JSX In Depth” page
Merge pull request #809 from passy/patch-1

Add gulp-react to Helpful OSS Projects
Merge pull request #643 from spicyj/scroll-pos

Fix scrollLeft/scrollTop warning in latest Chrome
Merge pull request #747 from spicyj/currentTarget

Set currentTarget on synthetic events