GSM-7 National language shift tables support

[belousotroll]

check this out: https://en.wikipedia.org/wiki/GSM_03.38#National_language_shift_tables

[mitshell]

Please note a new repository has been setup, including new fixes. This repo will keep being maintained : https://github.com/pycrate-org/pycrate.

SMS content encoding and decoding is defined in https://github.com/pycrate-org/pycrate/blob/master/pycrate_mobile/TS23038.py. Yet only the default 7 bit encoding is supported. Any contribution or help would be much appreciated here.