Add a model generator

Usage:

```
% vnet/tools/generator hoge -ne

create: /Users/jemiam/work/openvnet/vnet/lib/vnet/models/hoge.rb
create: /Users/jemiam/work/openvnet/vnet/lib/vnet/model_wrappers/hoge.rb
create: /Users/jemiam/work/openvnet/vnet/lib/vnet/node_api/hoge.rb
create: /Users/jemiam/work/openvnet/vnet/lib/vnet/endpoints/1.0/hoges.rb
create: /Users/jemiam/work/openvnet/vnet/lib/vnet/endpoints/1.0/responses/hoge.rb

-------------------------
Autoload configurations
-------------------------

file: /Users/jemiam/work/openvnet/vnet/lib/vnet/lib/vnet.rb

module Vnet
  module Models
    autoload :Hoge, 'vnet/models/hoge'
  end

  module ModelWrappers
    autoload :Hoge, 'vnet/model_wrappers/hoge'
  end

  module NodeApi
    autoload :Hoge, 'vnet/node_api/hoge'
  end

  module Endpoints
    module V10
      module Responses
        autoload :Hoge, 'vnet/endpoints/1.0/responses/hoge'
        autoload :HogeCollection, 'vnet/endpoints/1.0/responses/hoge'
      end
    end
  end
end

-------------------------
Endpoint configurations
-------------------------

file: /Users/jemiam/work/openvnet/vnet/lib/vnet/endpoints/1.0/vnet_api.rb

module Vnet::Endpoints::V10
  class VnetAPI < Sinatra::Base
    load_namespace('hoges')
  end
end

```

Options:

d: delete mode
n: includes node api
e: includes endpoint