Add a loading.gif on submit form widget

Ajout d'une image de loading visible à la soumission du form généré par
le widget.
Merge branch 'pr_13' into development
Merge branch 'development' of https://github.com/mailjet/wordpress-mailjet-plugin-apiv3 into development
Put index file for protection in case someone try to access the directory

Put index file for protection in case someone try to access the
directory
RedMine issue #1751 - Show successful message when we save the changes of the Mailjet's widget, also if the contact list is not selected fro
Version is changed to 3.1.4
Merge development with master

Merge development with master