Add changeHandler opt

I just added `changeHandler` option.

It should be a function with the following signature:

    changeHandler (timezoneName, countryName, offset)

and gets called upon timezone change.

That makes `timezonepicker` much easier to use on <a href="http://developer.android.com/reference/android/webkit/WebView.html#addJavascriptInterface(java.lang.Object, java.lang.String)"><strong>Android</strong></a>/[**JavaFX**](https://blogs.oracle.com/javafx/entry/communicating_between_javascript_and_javafx) **WebView** or similar environments via a bridge.

Also I used [uglifyjs](https://github.com/mishoo/UglifyJS2) with default options for minification. Let me know if there's a better minifier/configuration that I should use.