Add support for streams / pipes

I love the simplicity of your avconv library, but I was sorely lacking support for pipe:0 input and pipe:1 output.

This commit adds that.

All the informational data is now emitted as a 'message' event,
while the converted data is now using the 'data' event.

The 'end' event has also been renamed to 'exit', because the 'end' event makes the stream unreadable.