add "timeout" optional property to OpenTok constructor

[aoberoi]

timeout is a useful option to configure the http layer.

[aoberoi]

Through discussion with the Product team, we've come to an agreement that a configurable 'timeout' is not in fact supported by any interesting use cases. Instead, we will be adding a default timeout of 20 seconds, so that requests do not indefinitely hang (as of node 0.10.x, there is no timeout by default).