Print debug logs NODE_DEBUG=request in environment

Used the same kind of debugging technique that's all throughout core.

It's fast, nodey, and extremely useful when tracking down bugs.

The next commit will fix the bugs I tracked down using this method.
Flow data in v0.10-style streams

Note: Request is *not*, at this point, presenting a streams2
API.  However, with this patch, it does not leave responses in
a paused state indefinitely, by taking care to consume every
request that it receives.
Merge pull request #473 from isaacs/v0.10-compat

V0.10 compat