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/src/Core/DiagonalMatrix.h

@@ -47,7 +47,7 @@ struct ei_traits<DiagonalMatrix<CoeffsVectorType> >
     ColsAtCompileTime = CoeffsVectorType::SizeAtCompileTime,
     MaxRowsAtCompileTime = CoeffsVectorType::MaxSizeAtCompileTime,
     MaxColsAtCompileTime = CoeffsVectorType::MaxSizeAtCompileTime,
-    Flags = CoeffsVectorType::Flags,
+    Flags = CoeffsVectorType::Flags & ~VectorizableBit,
     CoeffReadCost = CoeffsVectorType::CoeffReadCost
   };
 };