replace concat-stream with bl, fixes #251

quick sanity check from someone for this please and I'll do a patch release, the arguments change to concat-stream is kind of annoying and it doesn't deal with `err` properly anyway while **bl** does.
it's not a buffer, it's a pseudo-buffer, a "buffer list", that acts like a buffer but isn't actually one. i.e. the chunks of the original stream are kept separate to defer or remove any perf costs depending on what you're doing with it, the `slice()` is a way of lazily concatenating them into a single proper buffer