Make plugin translateable

I've gone through the widget code and made all output strings translateable, as we needed this in German for a project.

English is the default, and I've included a German translation (work in progress ... it's not my native language).

I've also added a `composer.json` file, so that the package could be installed as a [wpackagist](http://wpackagist.org/) file ... but if my changes ever make it into the code that's served up by WP, then this file is moot as the wpackagist process will handle it for you.

Enjoy!