Writes a value to the Stream. If the Stream is paused it will go into the
Stream's incoming buffer, otherwise it will be immediately processed and
sent to the Stream's consumers (if any). Returns false if the Stream is
paused, true otherwise. This lets Node's pipe method handle back-pressure.
You shouldn't need to call this yourself, but it may be called by Node
functions which treat Highland Streams as a [Node Writable Stream](http://nodejs.org/api/stream.html#stream_class_stream_writable).