Further cleanups of the Java bindings:

* Properly document the java bindings (mainly, add them to the Doxygen)

* No need for a ParallelTask class: they are handled exactly the same
   way than regular task, except at creation time.
  Thus giving Task a new constructor to build parallel tasks, and
   removing every other bit related to ParallelTask.java

* The Channel object was wrong: in MSG, channels are just like ports
   on each host. In jMSG, they were global tags containing all hosts.
  I just killed them.
  Task receiving methods are static Task methods.
  Task sending methods are attached to the host to which you want to
   send.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@5059 48e7efb5-ca39-0410-a469-dd3cf9ba447f