adds benchmark tool

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "3rd/gtest"]
 	path = 3rd/gtest
 	url = https://github.com/google/googletest.git
+[submodule "3rd/gbench"]
+	path = 3rd/gbench
+	url = git@github.com:henrydcase/benchmark.git

3rd/gbench

@@ -0,0 +1 @@
+Subproject commit e45fcc64e02489f718df499a162b41f742a1b7e5