Add init events for components

An event, ``'ons-component:init'``, will be fired when a component is initialized.
A help function, ``ons.eventToComponent(event)``, can be used to extract the component from the event object.

e.g.
```javascript
document.addEventHandler('ons-navigator:init', function(e) {
  var navigator = ons.eventToComponent(e);
  navigator.pushPage('some_page.html');
});
```