Fix proposal for #222 (support for multi-db)

I'm not very proud of the class variable but I don't see an easy way to get around this. Do you ?

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When creating a subclass of Globalize::ActiveRecord::Translation we
actually now create 2 classes.

One, abstract, inherits from Globalize::ActiveRecord::Translation and
establish a connection to the same database than the related model.
The second one inherits from the first one and acts like before.