support i18n

[Yuki-Inoue]

This PR makes ActiveHash support i18n if I18n is available.

With this fix, model's model_name will be configurable by locale file, for example:

ja:
  activemodel:
    models:
      country: "国" # Country.model_name.human # => "国"

[syguer]

Thank you for your contribution. Sounds good!
Could you write test codes and write short document to README.md?
If you are busy, I'll help these job 👍

[Yuki-Inoue]

@syguer Thanks for the review. The point out you've made is fixed with the next commit.

About the document/testing, I'll try. Document is relatively easy, whereas the test it might take some time for me to figure out how to setup... (all the locale setup usually made by rails, I find it not obvious how to write tests...)

[Yuki-Inoue]

README is updated.

[yhirano55]

It looks nice work to me 👏 I would like to know why this pull request is not merged.

[Yuki-Inoue]

From where I remember, I was asked to write tests.
I tried to do so, but had trouble; I couldn't figure out how to write test for this kind of feature.

I would be glad if anyone can help...