Only support Swift 4.0.3

Thanks to @Davidde94.

Add Swift 4 Support

Swift4 (#53)

* Add Swift 4 support

* add swift 4 support

* change swift version

Fix message-id, revert previous breaking API changes

Many thanks to @leanne63 for her work on this!

• Mail structs no longer require a hostname for initialization
• removed makeMail() function in SMTP struct

Improve connect with timeout

NOTE: The timeout parameter in initializing an SMTP handle is now a UInt.

Attempting to connect to a server should now correctly timeout and return an error when appropriate.

Mail objects require a hostname to init

[See #41]

Breaking API change for initializing a Mail object:

• Initializing a Mail object now requires a hostname to be provided.

New API for the SMTP struct:

• makeMail(): create a Mail object. It's hostname will be set to the SMTP struct's hostname.

SSL and additional headers changes

• SSL
  • If an SSL connection is required, Swift-SMTP will automatically attempt to upgrade the connection without a need for the user to specify an SSL configuration
  • With this default configuration, no backing certificates are used for the client's side of the SSL handshake with the SMTP server
  • View docs on the SSL struct for more configuration options, like adding a certificate chain, etc.
• Additional headers:
  • CONTENT-TYPE, CONTENT-DISPOSITION, CONTENT-TRANSFER-ENCODING headers added in additionalHeaders will now be ignored. Allowing them to be overwritten causes errors for some SMTP servers and mail content to render incorrectly.
    • CONTENT-TYPE will be set by Swift-SMTP--plaintext/html should render correctly in the body of emails. Attachments will also have their CONTENT-TYPE correctly set by Swift-SMTP.
    • For CONTENT-DISPOSITION, set the inline flag in the initialization of an Attachment if you need to customize this
    • Users should not need to change CONTENT-TRANSFER-ENCODING

Default headers for plain text mail content

• For plain text content of an email, these default headers are added:
  • CONTENT-TYPE: text/html; charset=utf-8
  • CONTENT-TRANSFER-ENCODING: 7bit
  • CONTENT-DISPOSITION: inline
• These headers can be overwritten by the user with the additionalHeaders dictionary
  • This is usually not recommended as any headers in additionalHeaders are applied to the entire email, so multi content-type emails will be rendered incorrectly. This will be noted in the docs.
• Additional changes to the other content types (mixed, alternative, related) will also be made so that the default CONTENT-TYPE header can be overwritten as well (but again, not recommended)

Dependency graph issue, plain text header issues

• added .git. to BlueCryptor url in Package.swift
• removed hard coded header in plain text content of emails

Change logging levels, headers can now be overwritten

• SMTPSocket logging level is now at debug level (you will no longer see the logs on verbose level)
• A Mail's or Attachment's additionalHeaders is now a dictionary. This means headers can be overwritten. Note that all header keys are capitalized.

Add Swift 4 support

swift-4.0-DEVELOPMENT-SNAPSHOT-2017-06-11-a