Use ISO 8601 UTC formatted dates in JSON messages instead of timestamp format

There has been discussion in the Vert.x group on the date format used in JSON messages. See: https://groups.google.com/forum/#!topic/vertx/a0yeLL23GyQ. 

With the standard Jackson ObjectMapper the format defaults to using timestamps.
However for JSON a format of ISO 8601 in UTC timezone is considered to be a best-practice (see for example the excellent webinar by Les Hazlewood on this subject http://www.youtube.com/watch?v=mZ8_QgJ5mbs).

Note: I didn't write testcode for the change in the ObjectMapper config...unfamiliar with the Gradle setup (used to Maven), so didn't get tests working properly and test in a working vert.x build. Maybe someone can add this.

(also used 'Organize imports' in Eclipse...sorry :)

Change will probably cause backward-compatibility issues....please review.