export ExtBuffer

[wemoloh]

Hi kawanet,

All this change does is add ExtBuffer to the module exports. I have a project that's currently using this workaround:

var ExtBuffer = msgpack.decode([0xd4, 0, 0]).constructor;

...to enable me to manually create ExtBuffer objects for encoding. The addExtPacker functionality won't work for me because I may have an unknown number of subclasses with different constructor.name properties.

I'd be fine with leaving ExtBuffer undocumented, or documenting it with a 'use at your own risk' warning.

Thanks!
Bart