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.
At first I decided to typecast the key to `{Object}`, but since we do not expect applications to provide their own instance of a key, I think it is enough to tell the compiler that what we get as return type from `on` and `once` is the same that we provide as argument to `unByKey`.

Makes sense?