SMTP class for GSMSHIELD

[mgaman]

The SIM900 has more capabilities than normally advertised, namely SMTP and HTTPS support. To check whether your version of SIM900 supports SMTP, attach a terminal emulator and type in AT_EMAILCID=? If the reply ends in OK, you're supported. This is a work in progress. Currently ASCII mode is supported for Subject and Body. UTF-8 mode is supported but never tested. Mixing SSL and login names/ passwords with non-alphanumeric characters causes the SIM900 to reset. Similarily non-alphanumeric characters in https URLs exhibits similar behaviour. I need to fix that so I can develop a Telegram class. Your testing and feedback would be appreciated.