Remove using namespace std;

For historical reasons we had a "using namespace std;" in port.h. This
is generally a bad idea. So removing it and along the way doing a bunch
of cpplint cleanup.

Change-Id: Ia125601a55ae62695e247fb0250df4c6f86c46c6

internal/ceres/gradient_checking_cost_function_test.cc

@@ -48,6 +48,7 @@
 #include "gmock/mock-log.h"
 #include "gtest/gtest.h"
 
+using std::vector;
 using testing::AllOf;
 using testing::AnyNumber;
 using testing::HasSubstr;