Added onBWCheck method for the RTMP streaming

I got an ActionScript error when rtmp streaming was started in debugging.

```
Error #2044: Unhandled AsyncErrorEvent : 
text=Error #2095: flash.net.NetConnection was unable to invoke callback onBWCheck
error=ReferenceError: Error #1069: Property onBWCheck not found on flash.net.NetConnection and there is no default value.
	at com.videojs.providers::RTMPVideoProvider/initNetConnection()
	at com.videojs.providers::RTMPVideoProvider/play()
	at com.videojs::VideoJSModel/play()
	at VideoJS/onPlayCalled()
	at Function/http://adobe.com/AS3/2006/builtin::apply()
	at flash.external::ExternalInterface$/_callIn()
	at Function/<anonymous>()
```

According to the developers guide, it requires to be implemented.
http://help.adobe.com/en_US/FlashMediaServer/3.5_Deving/WS5b3ccc516d4fbf351e63e3d11a0773d56e-7ffa.html