v0.7.0: Reconsidering Methods.

• (breaking-change) Require helo_domain option when creating EMail::Message::Config object.
• (breaking-change) Change the 2nd String argument name of #sender, #return_path, #from, #to, #cc, #bcc and #reply-to methods of EMail::Message class from sender_name to mailbox_name.
• Allow taking an array of String or EMail::Address object by #from, #to, #cc, #bcc and #reply-to method of EMail::Message class to set multiple email address at once.
• Add EMail::Message#clear_** methods for From, To, Cc, Bcc and Reply-To headers to delete all email address set the header.
• Add EMail::Message#clear_custom_header method to delete custom headers with specific name.
• Cleanup code and docs.