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/Constants.java

@@ -33,5 +33,6 @@ public final class Constants {
         public static final String default_file_compression = "defaultFileCompression";
         public static final String pass_phrase_cache_ttl = "passPhraseCacheTtl";
         public static final String language = "language";
+        public static final String force_v3_signatures = "forceV3Signatures";
     }
 }