golang std lib: stmp no auth LOGIN

[exfly]

Our company's private mail server uses AUTH LOGIN. There is no such AUTH LOGIN in go std lib: stmp. Later people will deal with this problem repeatedly, which is very inefficient.

To fix this, submitted a mr #103

[xhit]

Your Pull Request is the auth in std library adapted for gomail. This package doesn't use any std package embedded, so adapt the auth.go file require others auth types like CRAM-MD5 and Plain Auth and adapt email.go file for use auth.go instead auth.go file in "net/smtp"

The package go-simple-mail https://github.com/xhit/go-simple-mail resolves this issue. Feel free to use the method in this package for implement auth LOGIN in jordan-wright email package