AWS support

  * adding Gemfile to ease dependency management
  * adding some much needed passthrough documentation
  * kill all running machines before ```rake test```
  * adding AWS support
    * examples/aws.rb - attach to a running instance, start a new instance and attach to it
    * lib/rouster.rb - implementing :passthrough => { :type => :aws }, sane defaults (:ami, :key_id, :min_count, :max_count, :region, :secret_key, :size, :ssh_port, :user)
    * plugins/aws.rb - AWS specific functionality
        * public:.aws_get_ip(), .aws_get_userdata(), .aws_get_metadata(), .aws_get_instance(), .aws_get_ami(), .aws_describe_instance(), .aws_bootstrap(),
        * ~private:  .aws_get_url(), .aws_up(), .aws_destroy(), .aws_status(), .aws_connect_to_elb(), .aws_connect(), .elb_connect(), .find_ssh_elb()
  * setting some sane ssh defaults and generalizing passthrough retries (Net::SSH :paranoid, :ssh_sleep_ceiling, :ssh_sleep_time)
  * intercepting vagrant() calls for .up(), .destroy(), .status() if :aws
  * cleaning up path_helper.rb
  * adding/fixing tests: functional/test_new.rb, functional/test_passthroughs.rb,