BugFix

My first step today was to fix the plonetrackback bug for deleting received trackback pings (issue 90).

After deleting a trackback ping, it gets uncataloged.  However, the way that the pings are stored in the catalog is rather convoluted and hackish.  As a result, the standard unindexObject call didn't work as expected, and instead also uncataloged the actual WeblogEntry instance to which the ping was attached.  I resolved this with a change to uncatalog_object(trackback.getPath()).