REFACTOR configure mailer for mandrill

Smtp configuration for mandrill has been set.

To install addon on heroku :

    heroku addons:add mandrill:starter

Two environment variables have to be set :

* MANDRILL_USERNAME
* MANDRILL_APIKEY

Documentation mentions that api key is automatically set up, but does not
say anything about how to provide user name (maybe it's a free field?).

Let us now if any further info is provided when installing the addon.

The full documentation can be found here :
https://devcenter.heroku.com/articles/mandrill