Update bundled gtest to latest version (1.3.0) and include it as a
nested autoconf package rather than as raw source.  This way we can
trivially update it again in the future.

Actually, this change doesn't even include gtest in protobuf's SVN.
Instead, we auto-download it when autogen.sh is invoked.  Note that
it will be included in release distributions, though.

TODO:
* Add a configure option to use the system's installed gtest rather
  than the bundled copy.  Apparently the gtest maintainers are working
  on some general-purpose autoconf macros which will do this
  automagically.
* Update MSVC project files.

diff --git a/CHANGES.txt b/CHANGES.txt
index e54e1fd..2ac9091 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -17,6 +17,9 @@
       Python:  Foo.BAR_BAZ_FIELD_NUMBER
     Constants are also generated for extensions, with the same naming scheme.
     These constants may be used as switch cases.
+  * Updated bundled Google Test to version 1.3.0.  Google Test is now bundled
+    in its verbatim form as a nested autoconf package, so you can drop in any
+    other version of Google Test if needed.
 
   protoc
   * --error_format=msvs option causes errors to be printed in Visual Studio