Rainbows! 0.97.0

We now depend on Unicorn 1.1.3 to avoid race conditions during
log cycling.  This bug mainly affected folks using Rainbows! as
a multithreaded static file server.

"keepalive_timeout 0" now works as documented for all backends
to completely disable keepalive.  This was previously broken
under EventMachine, Rev, and Revactor.

There is a new Rainbows::ThreadTimeout Rack middleware which
gives soft timeouts to apps running on multithreaded backends.

There are several bugfixes for proxying IO objects and the usual
round of small code cleanups and documentation updates.

See the commits in git for all the details.