Use Default Value for Undefined Props

Currently, default props as defined by `getDefaultProps` are only used if a prop is not specified (using `X in Y` check).

However, this makes it difficult for composing components to pass along the state of not being defined, for example:

  render: function() {
    // If `this.props.someProp` is not set, `InnerThing` should use the default value.
    return <InnerThing someProp={this.props.someProp} />;
  }

This changes the requirement for falling back to the default value to an undefined check (using `typeof X === 'undefined'`).
Add clickable anchors to docs headers

Closes #434
docs: remove OUTLINE
docs: Delete Mutation Events (onCharacterDOMModified)
Don't reset mouseDown in focus handlers

Focus fires after mouse down on initial click, so we lost the
flag when the user initially began dragging on the input.
fixed %d in invariant call
ReactTransitionGroup example: fix typo and logic bug in handleRemove
Fix ReactTransitionEvents detectEvents
Merge pull request #446 from brianr/master

ReactTransitionGroup example: fix typo and logic bug in handleRemove
Update API docs for unmountAndReleaseReactRootNode
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.