Added Custom SoftwareSerial

Rebased SerialGSM to use a custom SoftwareSerial implementation
Enabled Interrupts

Enable interrupts in FlowControlSoftwareSerial
Ported in basic flow control

Portions obtained from the official Arduino GSM Library.
https://github.com/arduino/Arduino/blob/master/libraries/GSM/GSM3SoftSerial.cpp
Enable modem software flow control

Send AT+IFC=1,1 to force the module into software flow control mode
Working Flow Control

Removed unnecessary code and increased buffer size
Clear serial buffer while booting

Calling WaitResp for a SIND 10 ensures that the serial buffer is
constantly cleared.