Send core emails

Allow developers to use standard rails mail configuration (in config files) by setting `:override_actionmailer_config` to false without sending spree core emails (e.g. order confirmation). This is useful e.g. in the    case where devs have opted to use an external mail API such as Mandrill    for store-related emails but still want to use ActionMailer in other    parts of their app.

I named the config preference `send_core_emails`. I'm open to other naming if preferred. Also, I put a small spec for this in `order_mailer_spec.rb`, not sure if that's the best location for such a spec.

Finally, I'm aliasing the `#mail` action in `Spree::BaseMailer`, a subclass of `ActionMailer::Base` from which all other spree mailers inherit. Not sure if this might be considered dangerous/harmful.