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
expandedBufferTime(=5) is only used for setting buffer time second after the initialBufferTime. This means that the bufferTime is updated when NetStream.Buffer.Full event happens, see below.
```
1 -> 5 -> 10 -> 20 -> 40 -> ...
```
The reason why using expandedBufferTime is to increase bufferTime quickly. As long as I debugged, NetStream.Buffer.Full event doesn't happen frequently. If we don't use expandedBufferTime, bufferTime is updated per NetStream.Buffer.Full event like this.
```
1 -> 2 -> 4 -> 8 -> 16 -> ...
```
Of course it's working, but it's not efficient as increasing bufferTime.