Maintain a single png or tcp stream in client

Here is a first partial solution for issue #43 that is sufficient to solve some my problems. It should not break the current API and issues a warning for deprecation on the createPngStream method. 

However it is not a a complete solution, since the PngStream will instantiate its own TcpVideoStream, thus disconnecting any other stream obtained via getTcpVideoStream.

In addition, using any other module which directly manage its own tcpstream (e.g. node-dronestream) will also conflict and break everything. 

Feedback/comments welcome.