Rainbows! 3.0.0 - serving the fastest apps to slow clients faster!

There is one incompatible change: We no longer assume application
authors are crazy and use strangely-cased headers for "Content-Length",
"Transfer-Encoding", and "Range".  This allows us to avoid the
case-insensitivity of Rack::Utils::HeaderHash for a speed boost on the
few apps that already serve thousands of requests/second per-worker.

:Coolio got "async.callback" support like :EventMachine, but it
currently lacks EM::Deferrables which would allow us to call
"succeed"/"fail" callbacks.  This means only one-shot response writes
are supported.

There are numerous internal code cleanups and several bugfixes for
handling partial static file responses.