Update frame code

Some update of the frame code:
 - methods `childFramesCount`, `childFramesName` and `switchToChildFrame` are deprecated. Replace them with their new synonyms;
 - I need to play with the content of an iframe for more than one step, so I exposed the `switchToFrame` method;
 - also, I need my clients scripts into the frame, so I injected them;
 - I like events, so I added a `frame.changed` event.To be consistent, maybe should I also encapsulate `switchToParentFrame` and `switchToMainFrame`, to also emit this event ?