Fix conversion warnings/errors

See #411.
  http://paste.debian.net/378673/
diff --git a/include/json/writer.h b/include/json/writer.h
index f94aa1f..180ab68 100644
--- a/include/json/writer.h
+++ b/include/json/writer.h
@@ -238,8 +238,8 @@
   ChildValues childValues_;
   std::string document_;
   std::string indentString_;
-  int rightMargin_;
-  int indentSize_;
+  unsigned int rightMargin_;
+  unsigned int indentSize_;
   bool addChildValues_;
 };