Admin site

@weskendall This pull request provides a clean interface to send emails through the admin site.

The focus is on a simple useable interface for sending emails. As such, it removes some flexibility in favor of having a user of the admin site able to send an email without understanding anything about the `entity_emailer` internals.

It also provides a management command that must be run to set up some database properties for the admin site to work correctly. This management command can be run as:

     python manage.py entity_emailer_admin_setup

Alternatively, a site that has some way of populating initial data can populate an `EmailTemplate` object and a `Source` object with names corresponding to those expected by the admin (defaulting to 'html_safe' and 'admin' respectively).