RFR Cp Bus Order

issue #37 
Nice job :)  

Since this method is essentially doing something to just a single `Bus` object at a time, it's better to make it an instance method.  That way, you don't have to pass in `number` as a parameter, and it makes more sense from an OO standpoint -- you're asking an object about itself, not the class about one object of that class.

```ruby
  def order_number
    if number[0].match(/\d/)
      if number.scan(/\d/).size == 1
        "00" + number
      elsif number.scan(/\d/).size == 2
        "0" + number
      else
        number
      end
    else
      number
    end
  end
```