Added doccomments from MDN to Javascript standard library stubs

This diff adds Scaladoc comments to the bulk of definitions (classes, objects, methods) in the ScalaJS standard library. These are under a distinct license (CreativeCommonsAttributionShareAlike) and are thus demarcated as "MDN", with a bit of license blurb at the top of the file to elaborate on the license. This explicit demarcation is the same technique that Google's dart docs and the WebPlatform docs use to serve up the MDN content.

It's not perfect, but it gives a lot of help right at the place you're likely to be looking at when you need it (i.e. at the definition that you don't understand) rather than asking you to start googling all over the internet. It also provides a degree of assurance that these functions and classes are actually real, functioning things, something that's missing when you face a wall of undocumented stubs.