Added support for ActionMailer custom delivery methods

We are using Amazon SES to send emails from one of our systems and wanted to use your gem to safeguard our non-production environments.

Since the mail gem's delivery methods were being used, this resulted in the following error when we tried configuring:

`NoMethodError: undefined method 'new' for :amazon_ses:Symbol`

Our configuration looked something like this:

```ruby
config.action_mailer.safety_mailer_settings = {
  :allowed_matchers => [/foobar.com/],
  :delivery_method  => :amazon_ses
}
```

I've changed the implementation to use ActionMailer's delivery methods as the basis for the lookup.