provide translations: en, de and fr

[simevo]

see #17 and italia/spid-smart-button#16 (comment)

[michaeltieso]

For this to happen, the language packs will need to be located in /languages https://github.com/simevo/spid-wordpress/blob/master/spid-wordpress.php#L15

I set the text domain to spid-wordpress. Should be easy to translate from here but might be best to wait until near the end of .2 so we can get the language done the first time and won't need to keep translating after every changes.