Better mail api

- [x] Class based test suite
- [x] Unify interface for `reply_to`, `from_addr` addresses
- [x] Unify interface for `to`, `cc`, `bcc` addresses
- [x] Fix `bcc` header behaviour, e.g. do not add bcc to message headers but add to recipients list
- [x] Sanitize address input, e.g. remove `\r`, `\n`, etc.