I saw this one: http://www.ebay.com/itm/Digital-Light-Intensity-Sensor-Module-BH1750FVI-Stable-For-Arduino-TW-/131903322284?hash=item1eb60cf8ac:g:t1MAAOSwV0RXqVe9

This one looks better for outside use I think, because the sensor itself is more protected for water.

Think I have fixed it :)

I found this topic: https://forum.mysensors.org/topic/4680/mysensors-2-0-ethernet-gateway-atmega-w5100-restart-all-time/5

The answer for me was to downgrade the Arduino AVR Boards to version 1.6.11 (Using Tools/Boards/Boardsmanager) ... It seems there is a bug in the newer release of this tool and the W5100. So hopefully this issue can be fixed, so that in newer releases this is OK. Maybe it is wise to add this to the Ethernet Gateway Page when using a W5100 and Mysensors 2.0 ;)