Address some of the comments on CGNR patch

- Rename BlockDiagonalPreconditioner to BlockJacobiPreconditioner
- Include the diagonal in the block jacobi preconditioner.
- Better flag help for eta.
- Enable test for CGNR
- Rename CONJUGATE_GRADIENTS to CGNR.
- etc.

include/ceres/types.h

diff --git a/include/ceres/types.h b/include/ceres/types.h
index ea41b4d..b83a266 100644
--- a/include/ceres/types.h
+++ b/include/ceres/types.h
@@ -83,12 +83,8 @@
   // problems.
   ITERATIVE_SCHUR,
 
-  // Symmetric positive definite solvers
-
-  // This is not meant for direct client use; it is used under the
-  // hood while using ITERATIVE_SCHUR.  Once there is a decent
-  // preconditioner, this will make sense for general sparse problems.
-  CONJUGATE_GRADIENTS
+  // Conjugate gradients on the normal equations.
+  CGNR
 };
 
 enum PreconditionerType {