Changed checking for correct sender.

Changed checking for correct sender.
RedMine issue #1817

The rest of the string are translated to german, french and spanish. In
RedMine this issue is with number 1817.
The version is changed to 3.1.15

The version is changed to 3.1.15, including the changelog mentioning
that the version is changed because all strings are translated.
Translation of the iframe and corrections in i18n

Translation of the iframe is not available because we send to the
website an additional GET parameter called locale which sets the
language of the iframe's content; Also small corrections are executed
for the translations from i18n folder;
Merge pull request #23 from mailjet/development

Merge the development and master branches