Add ability to explicitly set the backend

This should fix issue #14.

Here is the example usage

```
require 'rack-proxy'

class CwsProxy < Rack::Proxy
  def initialize
    super :backend => MyConfig.backend_uri
  end

  def rewrite_env(env)
    env["HTTP_X_FORWARDED_HOST"] = env["HTTP_HOST"]
    env["HTTP_HOST"] = MyConfig.backend_uri.host

    env
  end

  def rewrite_response(triplet)
    status, headers, body = triplet
    headers.delete("status")
    triplet
  end
end


```