Add @Override annotation to generated Java code where appropriate.

Patch from Brian Atkinson <nairb774@gmail.com>.

diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index a22cda4..21c83c6 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -47,3 +47,5 @@
   Dilip Joseph <dilip.antony.joseph@gmail.com>
     * Heuristic detection of sub-messages when printing unknown fields in
       text format.
+  Brian Atkinson <nairb774@gmail.com>
+    * Added @Override annotation to generated Java code where appropriate.