feat: Add rawJsonString()

[awwa]

When I use smtpapi-java with JavaMail, it is better to handle unescaped JSON string, because JavaMail has ability to fold and encode the custom headers.
If the x-smtpapi header is longer than 1000 characters, we have to fold it(add line break per 1000 characters).
http://docs.oracle.com/javaee/6/api/javax/mail/internet/MimeUtility.html#encodeText(java.lang.String)
http://docs.oracle.com/javaee/6/api/javax/mail/internet/MimeUtility.html#fold(int, java.lang.String)

[awwa]

The usage for new method will be like this:

SMTPAPI smtpapi = new SMTPAPI();
smtpapi.setTos(tos);
smtpapi.addSubstitutions("name", names);
smtpapi.addCategory("category1");

MimeMessage message = new MimeMessage(mailSession);
message.setHeader(
      "X-SMTPAPI",
      MimeUtility.fold(76, MimeUtility.encodeText(smtpapi.rawJsonString()))
);

[eddiezane]

Thanks @awwa! You're the man as usual buddy!

[awwa]

@eddiezane ULTRA FAST! Thank you so much!!!