Trailers part deux

[mnot]

This is the continuation of the HTTP trailer work -- exposing them on the incoming side.

I've chosen to expose them as a .trailers property, as it seems the most straightforward approach.

Thanks!

[mranney]

I always thought that trailers sounded really crazy, but now that I see the code, this seems pretty reasonable and useful.

[ry]

landed in 1b24fc6

[ry]

thanks Mark!