additional JsAPI: loadVideo()

Hi, first of all thanks for doing a 100% open source flv player (source and development environment). 

For a project I work on I need to keep the player in place but want to play another file, for that I added loadVideo() - I am not sure if my code is clean, as I saw you have something like _firstLoad, not sure if I need to reset it, I just call _player.load() in JsApi.hx - and in my case it works (made a few tests).

This means also, all the further complexity like playlist now can be implemented in JS.