Added Contributed Platforms.

- TargetPackage / TargetPlatform / TargetBoard are now interfaces
- Contributions installed are detected during init time
- Tools must be referenced through "path" property (automatically set
  by the IDE to the contributed tool path)

app/src/processing/app/Base.java

@@ -91,6 +91,15 @@ public class Base {
 
     splashScreenHelper.splashText(_("Loading configuration..."));
 
+    try {
+      guardedMain(args);
+    } catch (Throwable e) {
+      e.printStackTrace(System.err);
+      System.exit(255);
+    }
+  }
+  
+  static public void guardedMain(String args[]) throws Exception {
     BaseNoGui.initLogger();
     
     BaseNoGui.notifier = new GUIUserNotifier();