Add missing Subject constructors and Subject classes from RxJava to RxScala

Also copies RxJava JavaDoc to ScalaDoc for each Subject as more visible documentation in Scala.
You should not use html tags in scaladoc, because for most of what you want to do, there's a better markdown-like version. For instance, if you use `{{{...}}}` instead of `<pre>...</pre>`, you'll get syntax highlighting. The best way to learn scaladoc is imho by example, such as by looking at any [class in the Scala standard library](https://github.com/scala/scala/blob/2.11.x/src/library/scala/collection/TraversableLike.scala).