Allow more than one callback to be register to NetworkInterfaces.
This introduces new APIs:
void NetworkInterface::add_event_listener(...);
void NetworkInterface::remove_event_listener(...);
Which internally calls interfaces attach() functions.
New classes covered with unit tests: SocketAddress, EthernetInterface, EMACInterface, WiFiAccessPoint.
Also added missing namespace in front of mbed::Callback usages.
EMACInterface is covered by the EthernetInterface unit tests.