Dispatch documention

- Updated the existing javadoc.
- Added javadoc to the new classes in `gwtp-dispatch-common-*` and `gwtp-dispatch-rpc-*`.
- Added javadoc to `gwtp-dispatch-rest`

An update of the wiki page can be found here: https://github.com/ArcBees/GWTP/wiki/Rest-Dispatch-1.1

@christiangoudreau @branflake2267 @meriouma @spg