Add java.util.Date

Simple java.util.Date implementation.

Note: Our Date.parse is less restrictive currently than Java's Date.parse, since it defers to the JavaScript Date constructor.
Please don't change the indentation. We use 4 spaces inside Seqs for settings. Also, we'd prefer you filter the option out (rather then defining separate `testSettings`), it reflects the intention more closely and is DRYer. See: https://github.com/scala-js/scala-js/blob/f3e77a83bc9c9aeac7f5f4382bbedbc7e701ff69/project/ScalaJSBuild.scala#L284 for an example.