added inverse_of to the relationships

Hi everyone,

I added the :inverse_of to the relationships between the translations and the globalized_model. This way you can for example access the globalized_model from the translations even before the translated model is saved.

I for example have a model like this

```ruby
class Page

  translates :path, :title
  accepts_nested_attributes_for :translations

  class Translation < Globalize::ActiveRecord::Translation

    before_validation :calculate_strings

    def calculate_strings
      # do something depending on globalized_model
    end
  end
end
```

which will currently not work when saving new pages.