Gsm network detection

Implemented network connectivity detection using pattern-matching on the GSM module.  I've tested this and have seen reports send consistently every minute for half an hour (NB: there was still one missed report - investigating that now).

Note that this implementation necessitated locking the MIB bus while waiting for registration.  This is unfortunate, but would be solved by asynchronous RPCs.

Also improved GSM SMS Stream robustness and error handling.