Allow to use email in an Elixir project

Thank you for this project, it makes it so easy to integrate with Mailgun :smiley:.

It can't be used as a dependency in an Elixir project as is though (http://elixir-lang.org). Elixir built in dependency management tool (`mix`) requires a `rebar.config` file (Cf. http://elixir-lang.org/getting_started/mix/1.html#toc_10).

This pull request adds a basic `rebar.config` file and updates the README file with instructions for using email in an Elixir project.