Irregular use of F() macro.

The F() macro is used only sometimes. The RAM load would be decreased if it were to be used more regularly.

Example:
```c++
String GSMSimTime::syncFromServer() {
	gsm.print("AT+CNTPCID=1\r"); //Bad
	_readSerial();

	gsm.print(F("AT+CNTP\r")); //Good
	_readSerial();
	//delay(50);
	_readSerial(30000);
```
Was it just forgotten, or is there something preventing it's use?