Add USSD method to gsmmodem.py
By this method you can easily run an ussd command. It will returns the command's result.
Add serial_for_url support
By this feature you can access a modem on tcp/ip by 'tcp' argument.
Raise related exceptions while connecting, sending and other failures
Try to send again if one part of multipart sms failed;
It tries 5 times and wait 3 seconds between every try. Also wait 3 seconds between sending each part of multipart sms.
Merge branches 'master' and 'ussd'