Escalate missing error when :raise is true

Before ec16ba75a5493b9da972eea08bae630eba35b62f,
ActionView::Helpers::TranslationHelper#translate has raised errors with
specifying options[:raise] to true.

This should work by this fix:

     begin
       t(:"translations.missing", raise: true)
     rescue I18n::MissingTranslationData
       p :hello!
     end

Conflicts:
	actionpack/CHANGELOG.md
Rails config for raise on missing translations

Add a config to setup whether raise exception for missing translation or
not.

Conflicts:
	actionview/CHANGELOG.md
	actionview/lib/action_view/base.rb
	guides/source/configuring.md
	railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt
Merge pull request #13 from square/backport-i18n-stuff

Backport i18n stuff