API clean-up and Risky::ListKeys module extraction

* Remove ActiveRecord (AR) like interface. If you want Risky to quack like AR, you'll need to extend it a bit (follows an example). You would possibly need that If you have associations from Risky models to AR models.

```ruby
class RiskyModel < Risky
  class << self
    # Make Risky quack like ActiveRecord
    alias_method :find_by_id, :find
    alias_method :find_all_by_id, :find_all_by_key

    def index2i(name, opts = {})
      opts.reverse_merge!(:finder => :find_by_id)
      super(name, opts)
    end
  end
end
```

* Extract unsafe operations into ListKeys module. To use these methods, you'll need to include Risky::ListKeys.

* Change `==` and `===` methods to have more semantic meaning. See [AR ==](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/core.rb#L282) for an example.