Site config updated with mailer settings

Updated the Site.config loader to include mailer settings. All site
configurations now being pulled from config/site.yml. Site.yml is not
stored in the repository. Example site.yml file included.

Site loader uses OpenStruct to import settings.
Factor out ENV calls. Includes update to admin basic auth

Basic authentication for /accounts must have a username and password to
continue. Check if both Site.admin_user and Site.admin_pass exist. If
not, throw an error.

Factored out remaining ENV calls. Mongoid.yml called directly.
Removed mongoid.yml from repository

Use symlinks to set up mongoid.yml in production.