Add swagger-ui rake task & Rack app

https://www.pivotaltracker.com/story/show/78399060
FYI, there is a new `__dir__` operand in Ruby 2 that yields the current directory of the file. This can make it easier to specify root directories. Also, there is likely no reason to call `File.expand_path` or `__dir__` twice here. One thing you can do to is something like this...

```
module Swagger
  def root
    .... __dir__ ...
  end

  module_function :root
end
```

and then refer to the root path of the gem via `Swagger.root`.