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
If I understand your comments correctly, you have an escalating bufferTime value which is triggered by the initial bufferTime being filled (via the Buffer.FULL event trigger). Problem is, Buffer.full triggers irregularly based on network conditions. I will talk with @heff about how the HTML element handles this, but I would probably not agree with adding that unless he thinks otherwise based on the inconsistency in which this functionality would be used.

I do agree with extending bufferTime to a configurable value, and I would sign off on this PR should that be the extent of it. However I don't think tying an escalating bufferTime to buffer.full events internally is a good idea for the stock codebase. Especially if the HTML tech counterpart does not doe the same.

I will defer to @heff otherwise for a final signoff on this one.