Fire event when subtitle changes

[alexismo]

This changes solves issue #9. It makes a "subtitlechanged" event available every time the content of a subtitle is changed. The event contains information about the event, such as:

• the div where the text should go
• the video
• timecode at which the event happened
• content of the new subtitle

I needed this feature in some of my code. Thought you might want to integrate it.

[thomassturm]

Thanks!