Sm deprecate

This pull request deprecates SMTP. SMTP will be supported outside this library. 

We'll build an example that people who still want to use SMTP can use. 

It will use the soon to be created https://github.com/sendgrid/xsmtpapi-php to easily generate the X-SMTPAPI headers. 

This deprecation is also happening to the nodejs lib and go lib at the moment.