Compatibility Office 365 HTML emails

HTML emails on Office 365 are received as an attachment due to the mime multipart/mixed set as default. However, this does not cause any trouble on Gmail or other providers as long as I am aware.

The following commit consist in setting a multipart/alternative mime type in the header, and has been tested on 365.

I also applied php-cs-fixer (https://github.com/fabpot/PHP-CS-Fixer) to fix the code syntax on the module.