Adding LoRaWANStack class to control MAC and PHY

LoRaWANStack class is our controller layer on top of our
current MAC and PHY layer. It provides services to an implementation
of LoRaWANBase class.

It is a singleton class owing to the fact that the mac layer underneath
is not a class object. Instead, it uses the MAC via setting mib, mlme, mcps
requests and getting responses back from the mac layer using confirmations and
indications.

In essense this class is a special handle for
mac layer underneath which is predominantly reference design based.
In future we may refactor the LoRaMac.cpp code to make it object oriented
and cleaner.

At one end, it binds the application selected radio driver with the PHY layer
and at the other end it provides services to upper layers handling the mac via
well defined APIs.

For proper selection of a PHY layer, user must use Mbed config system.
For this purpose an mbed_lib.json is provided which can be overriden by the
user defined mbed_app.json. By default the EU868 band is selected as a PHY layer.
User must set relevant keys for the selected connection mechanism.

Author: Hasnain Virk