add experimental code for sparse matrix:

- uses the common "Compressed Column Storage" scheme
 - supports every unary and binary operators with xpr template
   assuming binaryOp(0,0) == 0 and unaryOp(0) = 0 (otherwise a sparse
   matrix doesnot make sense)
 - this is the first commit, so of course, there are still several shorcommings !

Eigen/src/Core/MatrixBase.h

@@ -55,6 +55,8 @@ template<typename Derived> class MatrixBase
 
   public:
 
+    class InnerIterator;
+
     typedef typename ei_traits<Derived>::Scalar Scalar;
     typedef typename ei_packet_traits<Scalar>::type PacketScalar;