Stream API for implementors

[nadako]

See http://nodejs.org/api/stream.html#stream_api_for_stream_implementors

I guess we can provide signatures for that by specifying private methods in externs.

[nadako]

I added documented private methods, but I'm worried about that objectMode option. It means that instead of strings and buffers, streams can read/write random objects which really goes contrary with strict typing.

Maybe we should not support that for now and think of some separate extern base classes for object streams.

[mikicho]

@nadako Does this issue still relevant?