concat does not propagate events

When two streams are concatted, events are not propagated do the resulting stream.

``` coffeescript
s1 = _ ["hello"]
s2 = _ ["world"]

s = s1 .concat s2

s.on "foo", (v) -> ... # Never invoked

s1.emit "foo", foo
```

Start at the concat entry point in Highland's stream implementation and reproduce the issue with the provided s1, s2, and foo event snippet. Trace how events are handled on the concatenated stream; done means an event emitted by s1 reaches the listener attached to s, without breaking concatenation.

Written by the indexing model from the issue text.