Japanese localization strings (messages.json)

[toshiakikoike]

Because I don't have permission to edit or pull request to develop branch, I edited ja's i18n file(message.json) by coping to my repository.
https://github.com/toshiakikoike/epub_file/blob/master/readium_test/ja/messages.json
(not all.)
If this json is OK, please murge to develop branch.

[danielweck]

Could you please clone / fork the readium-js-viewer repository, edit the messages.json file from the develop branch, and create a pull-request (all of which can be done from the GitHub user interface, no need for command line)
Thanks!