3.0 email config

Move Email configuration into 2 separate methods and no longer rely on Configure. Following the [proposal from earlier this week](https://github.com/markstory/cakephp/wiki/Config-methods) I've update the Email class. Transport and delivery configuration are now independent and follow the conventions started in `Cache` and `Log`.

I'm not entirely sure `useConfig()` is the correct method name, but I'm open to suggestions.