Add check for SIM

[Krolken]

@drgagi suggested in #2 that a check for inserted SIM would be a good idea. I agree so adding an issue to see if I can add it or if anyone else wants to do it.

Try AT+CIMI which return IMSI number located on SIM card. If this command returns error it means no sim card inserted or connectors are bad so module can not detect sim.