Use internationalization in ActiveRecord and pluralize for others

This PR is a more up to date version of https://github.com/amatsuda/kaminari/pull/309
Also, it adds support for internationalization of `entry_name` for ActiveRecord. I tried to implement it for DataMapper but failed, maybe someone else with more experience with DM can have a look?

This PR solves the issues I described in https://github.com/amatsuda/kaminari/pull/309 where only using `.pluralize` won't work for other languages than English. Instead we should make use of the !18n features.

Any thoughts about this?
Sorry. Actually, it was not `model.model_name` before. `model` is not defined in that class. That's why the travis build is failing now.