Include googletest as a submodule (#3993)

Add googletest as a submodule in third_party/googletest.

src/README.md

diff --git a/src/README.md b/src/README.md
index 3ae23fa..b553cf2 100644
--- a/src/README.md
+++ b/src/README.md
@@ -15,7 +15,6 @@
   * autoconf
   * automake
   * libtool
-  * curl (used to download gmock)
   * make
   * g++
   * unzip
@@ -30,6 +29,7 @@
 If you get the source from github, you need to generate the configure script
 first:
 
+    $ git submodule update --init --recursive
     $ ./autogen.sh
 
 This will download gmock source (which is used for C++ Protocol Buffer