Add getter/setter for Netstream.bufferTime to control buffering

The video-js-swf doesn't have the interface to get/set Netstream.bufferTime and have set bufferTime=1(second) internally. It's almost working if network bandwidth is enough for streaming. However, handling the bufferTime is useful for narrow bandwidth.

Adobe documents describes how to handle the bufferTime dynamically.
http://help.adobe.com/en_US/flashmediaserver/devguide/WS5b3ccc516d4fbf351e63e3d11a0773d56e-7feaDev.html#WS82FD33FA-43D8-4f7a-BEAE-E3950CB694A1Dev