Update initialize, start, etc to use kwargs

[nevans]

• update POP3.foreach, POP3.delete_all, and POP3.auth_only to forward all arguments (*arg, **kwarg) to POP3.start.
• update auth_only to only forward args to start and return true
• marked POP3.create_ssl_params with :nodoc: to discourage its use.
• update #initialize, .start, #start, etc to allow kwargs for port and apop,
• add the ability to set the basic configuration parameters to #initialize: ssl, open_timeout, read_timeout, and debug_output.
• add auth keyword param to start, with the ability to forward it as keyword params into the #auth kw args

[nevans]

These changes should all be backward compatible. Although they aren't strictly necessary for it, they simplified my SASL PR (TODO: make PR and insert number here).

If you would like me to, I can split this PR into several smaller PRs (for example, if you have concerns about some changes but others are okay).