plugins: Plugin for finding MNC length from IMSI

This plugin finds the MNC length from the IMSI using the valid MCC/MNC
pairs published by the ITU-T.
include: MNC length from IMSI driver interface
src: Auxiliar functions for MNC length plugins
include: Definitions for MNC lenght plugin helpers
src: Call to MNC lenght plugin when needed

The old workaround for finding the MNC lenght in case it is not found
in the SIM is removed, and instead the mnclength plugin is called.
build: Added mnclenght plugin and driver
Merge pull request #22 from alfonsosanchezbeato/mnc-length

mnclength plugin