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?
prefer to do `options[:count] || 1` over `#fetch`.

```ruby
hash = {}
Benchmark.ips do |x|
  x.report("#fetch")   { hash.fetch(:bar, 1) }
  x.report("#[] + ||") { hash[:bar] || 1     }
end
```

result:

```
Calculating -------------------------------------
               fetch    116800 i/100ms
             [] + ||    123675 i/100ms
-------------------------------------------------
               fetch  5327627.0 (±12.9%) i/s -   26046400 in   4.998840s
             [] + ||  7124577.9 (±11.4%) i/s -   35123700 in   5.012767s
```