Change the translation mechanism to use
`ActionView::Helpers::TranslationHelper`.

It has some advantages over only using `I18n.t`, such as the safe
translation if the key has the suffix `_html`.
Add support to html markup in I18n options.

Closes #764.
No need to require `action_view` here.
Add test case for keys prefixed with _html keys and a string value.
Translate the attributes prefixed with _html to keep the html markup.
Merge pull request #1024 from plataformatec/support_html_markup

Add Support to html markup in I18n options.