|#60 Add datagram support

* Added Vert.x datagram support along with tests
* Renamed `toJava()` methods to `asJava` to better represent that this
methods don't do conversion, but instead provide a Java view of the
Scala classes. This has been observed in other projects bridging between
Java and Scala, such as RxJava.
Added info on type annotations for handlers in overloaded methods