i18n support & Japanese translation

[japonophile]

This PR adds i18n (internationalization) support to the Kano profile app, including registration to Kano World.

Followed the same guidelines as in kano-greeter (see TRANSLATION.md).
The original code was already i18n-aware so the effort was minimum (just a few fixes here and there).

Also added Japanese translation, as an example.

I copied all files and tested it on my Kano after switching language to Japanese (see http://www.rs-online.com/designspark/electronics/knowledge-item/JPN-raspberry-pi-Japanese-version for details).

Let's share Kano awesomeness with non-English speaking children!

[Ealdwulf]

Thanks for this @japonophile!
While reviewing this, I realised that we need to adapt our internationalisation approach a bit for certain python modules. That's because they are used in more than one app, and the app will have its own po files. So for these modules we need to separately declare the _ function (or just use gettext). I've got a commit doing this here: b46e158

We're going to wait a while before merging this one as all the apps which depend on it will need testing, so we want to merge it at the start of a release cycle.