Made EmailServerCredentials username/password optional

[drfraser]

EmailServerCredentials username/password were mandatory settings which makes no sense if an INSECURE SMTP server (aka port 25) is being used - no need to login etc.

Also potentially possible are secured SMTP servers (e.g. through SSL client/server certificates) that require no login step and server.login() would have caused problems.

Closes #40

Example

    server = EmailServerCredentials(
        smtp_server="a.public.server",
        smtp_type=SMTPType.INSECURE
    ).get_server()

Checklist

• This pull request references any related issue by including "Closes #<ISSUE_NUMBER>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• This pull request includes tests or only affects documentation.
• Summarized PR's changes in CHANGELOG.md

[ahuang11]

Thanks! Few stylistic changes suggested.

[ahuang11]

I think we're close to wrapping this PR; just a couple more things:

1. adding back Optional to kwargs that default to None
2. Adding a simple unit test for testing not passing username / password.

Thanks for all your help!