Node task to generate externs for OpenLayers 3

This is useful for environments where devs want to use a compiled build of ol3 with Closure compiler, but without compiling ol3 together with their app.
I do not think you lose the type with this approach. If I had gone ahead with the type cast to `{Object}` you would have lost the type. But in this case, if you compile together with your app, the compiler will still complain if you pass something different to `unByKey()` than what you got from `on()` or `once()`.