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/CwiseBinaryOp.h

@@ -84,6 +84,8 @@ class CwiseBinaryOp : ei_no_assignment_operator,
     typedef typename ei_traits<CwiseBinaryOp>::LhsNested LhsNested;
     typedef typename ei_traits<CwiseBinaryOp>::RhsNested RhsNested;
 
+    class InnerIterator;
+
     inline CwiseBinaryOp(const Lhs& lhs, const Rhs& rhs, const BinaryOp& func = BinaryOp())
       : m_lhs(lhs), m_rhs(rhs), m_functor(func)
     {