[HORNETQ-1140] Provide a jar with native libs uploaded to Maven repo

* modify the layout inside the generate jars to have the library files
  in different directories depending on their architecture:

    $ jar tvf target/hornetq-native-2.3.0.BETA-SNAPSHOT.jar
    ...
    44272 Tue Feb 19 12:07:00 CET 2013 lib/linux-i686/libHornetQAIO32.so
    53350 Tue Feb 19 12:07:00 CET 2013 lib/linux-x86_64/libHornetQAIO64.so

With this layout, the jar content can be extracted as-is in AS7
org.hornetq module's directory
Merge pull request #866 from jmesnil/HORNETQ-1140_hornetq-native_layout

[HORNETQ-1140] Provide a jar with native libs uploaded to Maven repo