Updates to the Sparse unsupported solvers module.

* change Sparse* specialization's signatures from <..., int Backend> to <..., typename Backend>. Update SparseExtra accordingly to use structs instead of the SparseBackend enum.
* add SparseLDLT Cholmod specialization
* for Cholmod and UmfPack, SparseLU, SparseLLT and SparseLDLT now use ei_solve_retval and have the new solve() method (to be closer to the 3.0 API).

* fix doc

unsupported/Eigen/SparseExtra

@@ -27,6 +27,9 @@ namespace Eigen {
   * \endcode
   */
 
+struct DefaultBackend {};
+
+/*
 enum SparseBackend {
   DefaultBackend,
   Taucs,
@@ -34,6 +37,8 @@ enum SparseBackend {
   SuperLU,
   UmfPack
 };
+*/
+
 
 // solver flags
 enum {