added a "force V3 signature" setting similar to the GPG version, hopefully making APG useful for some special cases where that is needed

Update issue 39
Two new strings:
<string name="section_advanced">Advanced</string>
<string name="label_forceV3Signature">Force V3 Signatures</string>

"V3" is just "version 3" and should remain untranslated, both strings can be seen at the bottom of the preferences activity.

src/org/thialfihar/android/apg/PreferencesActivity.java

@@ -37,6 +37,7 @@ public class PreferencesActivity extends PreferenceActivity {
     private IntegerListPreference mMessageCompression = null;
     private IntegerListPreference mFileCompression = null;
     private CheckBoxPreference mAsciiArmour = null;
+    private CheckBoxPreference mForceV3Signatures = null;
     private Preferences mPreferences;
 
     @Override
@@ -210,6 +211,18 @@ public class PreferencesActivity extends PreferenceActivity {
                 return false;
             }
         });
+
+        mForceV3Signatures = (CheckBoxPreference) findPreference(Constants.pref.force_v3_signatures);
+        mForceV3Signatures.setChecked(mPreferences.getForceV3Signatures());
+        mForceV3Signatures.setOnPreferenceChangeListener(new Preference.OnPreferenceChangeListener()
+        {
+            public boolean onPreferenceChange(Preference preference, Object newValue)
+            {
+                mForceV3Signatures.setChecked((Boolean)newValue);
+                mPreferences.setForceV3Signatures((Boolean)newValue);
+                return false;
+            }
+        });
     }
 }