Add register notification command

This patch adds reg-notif command. It is used to register a callback
will be called when a notification/indication is received.

Note: This command does not enable notification/indication in remote
device. For that, you must write 0x01 byte in the respective client
characteristic configuration.
Add unregister notification command

This patch adds unreg-notif command. It is used to unregister the
previous callback that was set by reg-notif command. This way, the stack
will ignore any notification/indication received.

Note: This command does not disable notification/indication in remote
device. For that, you must write 0x00 byte in the respective client
characteristic configuration.
Add rssi command

That commands request RSSI from connected device.