ReactJS support for IE6/IE7 (reimplemented)

After discussion in [PL2133](https://github.com/facebook/react/pull/2133) I have made a lighter implementation of ChangeEventPlugin to support IE6/IE7 so it dont breaks existing behaviour of all other browsers.

Following changes were made to support old IE browsers IE6 and IE7
1. Algorithm to distinguish user event value changes and JS value changes on INPUT and TEXTAREA in ChangeEventPlugin was implemented for IE6/IE7 by using datetime diffs
2. String[0] replaced by String.charAt(0) in setInnerHTML
3. Checking renderNode.hasAttribute extended by IE6/IE7 compatible attribute check

To run react.js apps with IE6/IE7 you have to use [es5-shims](https://github.com/es-shims/es5-shim), [console-polyfill](https://github.com/paulmillr/console-polyfill) and [queryselector-polyfill](https://github.com/mtsyganov/queryselector-polyfill)