Move heading anchors 50px up to avoid nav bar

Fixes #447.

We do this by moving the actual anchored element up in the page without moving the actual text. (Apple uses a similar trick in their framed docs.) Now this looks a bit sillier on smaller screens but it's better overall.
Fix unmounting components mounted into doc element

If we are to unmount a component mounted into a document element we should
unmount it from document.documentElement and not from document.firstChild which
is a doctype element in this specific case.
Merge pull request #423 from andreypopp/master

Re-rendering components into a document leaks components
Make 'disabled' MUST_USE_ATTRIBUTE for compatibility with CSS [disabled] selectors.

When a ReactDOMComponent is created with the property `disabled: true` subsequently setting the property to `disabled: false` the HTML attribute `disabled="true"` was being left in the DOM.
Update everything for v0.5.1

Cherry-picked from f3db0006e832e24649278c9e9eab6b9ca2c80336, excluding
version changes.
blog post for 0.5.1
Forward Compatibility w/ WebKit & Blink

Newer versions of WebKit and Blink will support both `document.body.scrollTop` and `document.documentElement.scrollTop`. Therefore, implementing cross-browser compatibility by summing the two will no longer work.

This changes React to use `getUnboundedScrollPosition` so we get the fix and consistency in one change!

See: https://rniwa.com/2013-10-29/web-compatibility-story-of-scrolltop-and-scrollleft/
Add autoCorrect to list of supported DOM properties
Reorder DefaultDOMPropertyConfig

`autoCorrect` belongs with the non-standard properties list.
Bump populist version to v0.1.4.

The new version includes a fix for this IE8 issue reported by
@subtleGradient: https://github.com/benjamn/populist/issues/4

Closes #456.
Merge pull request #457 from benjamn/upgrade-populist

Bump populist version to v0.1.4
Switch utility function calls -> namespaced calls
Better `getUnboundedScrollPosition` for windows

Instead of using browser sniffing, `getUnboundedScrollPosition` can do
better and not have to depend on the `getDocumentScrollElement` module.