added task to check coding rules

I added the ant task checkJNodeRules that is based on pmd.

It checks the java source files and warn the developper of potential source of bugs.
Warning : As explained here in the third part (end of the page), pmd reports are only warning. The aim is not to change the source code to remove messages !

The result of the analysis is located in this directory : /JNode-All/build/reports (1 file per project : JNode-FS, JNode-Net ...)

For now, the task uses only rules provided by pmd. Later, if needed, we can add our custom rules.