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/array_utils_test.cc

@@ -38,6 +38,8 @@
 namespace ceres {
 namespace internal {
 
+using std::vector;
+
 TEST(ArrayUtils, IsArrayValid) {
   double x[3];
   x[0] = 0.0;