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

I'd rather support `username` and `login`, but promote the use of the former over the latter. Setting `login` could override `username`, for instance.