Added initial experimental support for explicit vectorization.

Currently only the following platform/operations are supported:
 - SSE2 compatible architecture
 - compiler compatible with intel's SSE2 intrinsics
 - float, double and int data types
 - fixed size matrices with a storage major dimension multiple of 4 (or 2 for double)
 - scalar-matrix product, component wise: +,-,*,min,max
 - matrix-matrix product only if the left matrix is vectorizable and column major
   or the right matrix is vectorizable and row major, e.g.:
   a.transpose() * b is not vectorized with the default column major storage.
To use it you must define EIGEN_VECTORIZE and EIGEN_INTEL_PLATFORM.

Eigen/Core

@@ -4,17 +4,26 @@
 #include <cstdlib>
 #include <cmath>
 #include <complex>
-#ifndef EIGEN_USE_CUSTOM_ASSERT
 #include <cassert>
-#endif
 #include <iostream>
 
+#ifdef EIGEN_VECTORIZE
+#ifdef EIGEN_INTEL_PLATFORM
+#include <emmintrin.h>
+#include <xmmintrin.h>
+#else
+#undef EIGEN_VECTORIZE
+#endif
+#endif
+
 namespace Eigen {
 
 #include "src/Core/Util.h"
 #include "src/Core/ForwardDeclarations.h"
 #include "src/Core/NumTraits.h"
 #include "src/Core/MathFunctions.h"
+#include "src/Core/PacketMath.h"
+#include "src/Core/Functors.h"
 #include "src/Core/MatrixBase.h"
 #include "src/Core/Coeffs.h"
 #include "src/Core/OperatorEquals.h"
@@ -42,7 +51,6 @@ namespace Eigen {
 #include "src/Core/IO.h"
 #include "src/Core/Swap.h"
 #include "src/Core/CommaInitializer.h"
-#include "src/Core/Functors.h"
 
 } // namespace Eigen