support for Rack hijack in request and response

Rack 1.5.0 (protocol version [1,2]) adds support for
hijacking the client socket (removing it from the control
of unicorn (or any other Rack webserver)).

Tested with rack 1.5.0.
ignore normal Rack response at request-time hijack

Once a connection is hijacked, we ignore it completely and leave
the connection at the mercy of the application.
Merge branch 'hijack'

* hijack:
  ignore normal Rack response at request-time hijack
  support for Rack hijack in request and response
unicorn 4.6.0pre1 - 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.