Revert missing translations [ci skip]

IO should be actual class name.
Merge pull request #857 from sinatrasapporo/japanese

Revert missing translations [ci skip]
Add in info about subclassing Sinatra::Application

This makes it more explicit for people that aren't familar with Sinatra
Merge pull request #858 from msgehard/sinatra_application_mentions

Add in info about subclassing Sinatra::Application