Add support for AT commands and USSD

[spidy0x0]

Hi buddy,it would be really great if you add support for AT and USSD commands to the project.
Any plans? :)

[jruipinto]

Hi. It already had that functions in past. I can make it happen again, maybe if you explain your idea better. Or you can make a pull request too :)

[spidy0x0]

Ok here,i want to be able to make USSD requests/sessions or send AT commands to my modem using this repository.But in your documentation,it shows only how to send SMS with this repository.
Is there any way i can achieve that? :)