Add Attributes formAction, formEncType, formMethod and formTarget

This resolves #2425.
[docs] Clarify when state may duplicate props
Own our fiddles so we can update them more easily
In `Transferring Props` guide, `Manual Transfer` section will lead an error

```c
var FancyCheckbox = React.createClass({
  render: function() {
    var fancyClass = this.props.checked ? 'FancyChecked' : 'FancyUnchecked';
    return (
      <div className={fancyClass} onClick={this.props.onClick}>
        {this.props.children}
      </div>
    );
  }
});
React.render(
  <FancyCheckbox checked={true} onClick={console.log}>
    Hello world!
  </FancyCheckbox>,
  document.body
);
```

When click `Hello world!` will lead a `Uncaught TypeError: Illegal invocation` error
Use String.charAt for character by index lookup

Using object property accessor for string character lookup breaks in
non-modern browsers.
Merge pull request #2651 from arnihermann/use-charat-not-string-index-markup-diff

Use String.charAt to lookup string characters by index
Merge pull request #2649 from cookfront/master

In `Transferring Props` guide, `Manual Transfer` section will lead an er...
Merge pull request #2639 from zpao/own-jsfiddle

Own our fiddles so we can update them more easily
Merge pull request #2635 from zpao/docs/props-state-clarification

[docs] Clarify when state may duplicate props
Use getSelection in IE where available

IE9+ has support for window.getSelection, but we’re still using
document.selection for IE9/10. Only use the old IE selection API if the
modern one is unavailable.
Merge pull request #2653 from salier/ie-selection

Use getSelection in IE where available
Support marginHeight and marginWidth attributes

marginHeight and marginWidth are used on iframes to set the default body margin inside the iframe.
Don't need MUST_USE_ATTRIBUTE for margin(Width|Height)
Merge pull request #2489 from cody/form

Add Attributes formAction, formEncType, formMethod and formTarget
Merge pull request #2658 from ianobermiller/margin-attrs

Support marginHeight and marginWidth attributes
Update 10.6-update.md

Spelling fix.
Merge pull request #2671 from benkeen/patch-1

Update 10.6-update.md
The same problem in issue #2648
Merge pull request #2672 from cookfront/master

The same problem in issue #2648
Fix new lint errors

We'll get to lint parity soon and then not need to do this.