add support for scope option on uniqueness validation for translated attributes

I submitted a pull request for this [earlier](https://github.com/svenfuchs/globalize3/pull/139), but there was a problem supporting Rails 3.0. Since we're not supporting 3.0 in the rails4 branch, I thought I'd resubmit it (with updates to make it work with minispec, etc.).

The tests should be self-explanatory but let me know if anything doesn't make sense. (I haven't looked at this for a while so I'll have to refresh my memory first though...)