Added: all missing functional tests

Beware, this PR adds a picture of a cute cat, along multiple functional tests.
What about making it in pure Node.js? I think you will know better the node api after that. But if you think that could be too boring, that's ok too.

https://github.com/felixge/node-combined-stream/pull/16/files#diff-b9195b4623543a592ab8c829e8518c79R24