Added preliminary support for DSN. Refs #4396

@markstory @AD7six thoughts on moving this into a Trait within the Core namespace and also adding it to Cache and Log?

- [x] Cache
- [x] Database
- [x] Email
- [x] Log

Rather than key juggling here (and anywhere else) perhaps the trait method take care or returning what cake's classes expect - i.e. user -> username, pass -> password.