feat #441 history enhancement

Singleton class aria.utils.History provides a cross-browser implementation of HTML5 native history API. With respect to the previous version, this one is more compliant with the standard behaviour:

- the onpopstate class event is not raised after a pushState or replaceState.
- the state property is available on the singleton and it corresponds to window.history.state.
- support for page reload and external navigation is supported also in browsers for which hash-based navigation fallback occurs (with the exception of IE7, for which the previous history is not kept after reloading, but only the current state is available).

However, the new version is backward compatible with the previous one. In order to disable backward compatibility, set aria.utils.History.isBackwardCompatible to false right after loading the class.