Representer rails generator

Make it possible to create representers with rails generators. This is related to #15.

Usage:

```
  rails g representer Band id name

  This will create a representer as follows:

  module BandRepresenter
    include Roar::Representer::JSON

    property :id
    property :name
  end

  You can pass :class and :extend property options:

    rails g singer id instrument:equipament:instrument_representer

  This will add property options:

    module SingerRepresenter
      include Roar::Representer::JSON

      property :id
      property :instrument, :class => Equipament, :extend => InstrumentRepresenter
    end

  You can also choose representer format with --format option

    rails g representer Band id name --format=xml

  module BandRepresenter
    include Roar::Representer::XML

    property :id
    property :name
  end
```