Issue #970: Rename features.h to json_features.h (#1024)

This patch fixes a build issue on CMake, presumably due to the new glibc
having a features.h include file. This patch renames our features.h file
to avoid a name collision.
diff --git a/include/json/forwards.h b/include/json/forwards.h
index 958b5bc..b0d981b 100644
--- a/include/json/forwards.h
+++ b/include/json/forwards.h
@@ -25,7 +25,7 @@
 class CharReader;
 class CharReaderBuilder;
 
-// features.h
+// json_features.h
 class Features;
 
 // value.h