Backward incompatible: Supercharged::Charge::Base is now module

Because of AR4 inheritance issues. Example:

```ruby
class Fruit < ActiveRecord::Base
  self.table_name = "fruits"
  self.abstract_class = true

  scope :good, -> { scoped }
end

class Apple < Fruit
  def capitalized_name
    name.upcase
  end
end

Apple.create!

# Rails 3
Apple.good # [Apple<...>]

# Rails 4
Apple.good # [Fruit<...>]
```