Merge pull request #11 from mailjet/development

Merge development with master
Merge pull request #12 from mailjet/development

Merge development with master
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