[WIP] added forecast.io node

added the forecast.io node. Looking for feedback/improvements
URL won't be set if the lat/lon are incorrect values coming into the node. In this case however you never call your callback and therefore you don't call node.send ever? In fact, I'm not quite sure how node works but never calling an expected callback smells like a possible memory leak too. So make sure you do call the callback. Set node.error and forward the input.