Add onDiscovered scan callback.

This adds a callback that is called when a device is initially discovered.
It is always called wether scanning in passive or active mode. This allows
for active scanning to receive device detection information in the case where
the scan response is not sent/received.

This callback is optional, the application may decide to implement it or not.

src/NimBLEAdvertisedDevice.cpp

@@ -33,7 +33,7 @@ NimBLEAdvertisedDevice::NimBLEAdvertisedDevice() :
 {
     m_advType          = 0;
     m_rssi             = -9999;
-    m_callbackSent     = false;
+    m_callbackSent     = 0;
     m_timestamp        = 0;
     m_advLength        = 0;
 } // NimBLEAdvertisedDevice