Use new Rack::Server thingy

[rtomayko]

All of Rack's option parsing and other rackup related stuff has been moved to a nice little Rack::Server class:

http://github.com/rack/rack/commit/5db5d4e7432d026a7bddf344f97e8facd09128d8

This should let us remove a huge swath of code from bin/shotgun.