incorporated Jose's magnetometer offset code into APM_Compass library so added a call to set the mag's declination (just to make it easier for people to add).

git-svn-id: https://arducopter.googlecode.com/svn/trunk@637 f9c3cf11-9bcb-44bc-f272-b75c42450872

Arducopter/Arducopter.pde

@@ -374,6 +374,7 @@ void setup()
   if (MAGNETOMETER == 1) {
     APM_Compass.Init();  // I2C initialization
     APM_Compass.SetOrientation(APM_COMPASS_COMPONENTS_UP_PINS_BACK);
+    APM_Compass.SetDeclination(ToRad(0.0));
   }
 
   DataFlash.StartWrite(1);   // Start a write session on page 1