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/CwiseUnaryOp.h

@@ -69,6 +69,8 @@ class CwiseUnaryOp : ei_no_assignment_operator,
 
     EIGEN_GENERIC_PUBLIC_INTERFACE(CwiseUnaryOp)
 
+    class InnerIterator;
+
     inline CwiseUnaryOp(const MatrixType& mat, const UnaryOp& func = UnaryOp())
       : m_matrix(mat), m_functor(func) {}