Email Support

This should add in full e-mail support, in terms of testing matchers/helpers, smtp settings, an e-mail preview gem, and mailcatcher for development. There is also several more additional changes.

* [x] Added `valid_email` gem.
* [x] Added `email_spec` gem, as well as its setup in RSpec.
* [x] Added `email_preview` gem.
* [x] Added .env and Environment Variable support for SMTP settings from development. **Production must still be configured manually depending on your individual setup.**
* [x] Made install_gem options[:group] more consistent with the Gemfile syntax.
* [x] Added Mailcatcher gem and related settings as to forwarded ports in the VM.
* [x] Updated the README.