Add periodic method to subsystems (#528)

Fixes #526.

wpilibj/src/shared/java/edu/wpi/first/wpilibj/command/Scheduler.java

@@ -198,6 +198,13 @@ public class Scheduler implements NamedSendable {
         ((ButtonScheduler) m_buttons.elementAt(i)).execute();
       }
     }
+
+    // Call every subsystem's periodic method
+    Enumeration subsystems = m_subsystems.getElements();
+    while (subsystems.hasMoreElements()) {
+      ((Subsystem) subsystems.nextElement()).periodic();
+    }
+
     // Loop through the commands
     LinkedListElement element = m_firstCommand;
     while (element != null) {

wpilibj/src/shared/java/edu/wpi/first/wpilibj/command/Subsystem.java

@@ -73,6 +73,13 @@ public abstract class Subsystem implements NamedSendable {
    */
   protected abstract void initDefaultCommand();
 
+  /**
+   * When the run method of the scheduler is called this method will be called.
+   */
+  public void periodic() {
+    // Override me!
+  }
+
   /**
    * Sets the default command. If this is not called or is called with null, then there will be no
    * default command for the subsystem.