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.