Composer / HTML message index

[s3rgiosan]

Hi,

This PR includes three things:

• Check first for $args['html'] and, if not present, fallback to $args['message']
  This is useful when using this plugin with wpMandrill.
• When a message is in HTML, use
  has a spacer instead of \n;
• Added a composer.json file to your plugin so that can be included using Composer;
• Check WP_DEBUG env variable for forcing email debug
• Added some dotfiles and tidied code