unicorn 4.6.0 - hijacking support

This pre-release adds hijacking support for Rack 1.5 users.
See Rack documentation for more information about hijacking.
There is also a new --no-default-middleware/-N option
for the `unicorn' command to ignore RACK_ENV within unicorn
thanks to Lin Jen-Shin.

There are only documentation and test-portability updates
since 4.6.0pre1, no code changes.