Rewrite of the tutorial.

1. Quicker starting point.
2. Better discussion of derivatives.
3. Better hyperlinking to code and class documentation.
4. New robust estimation example.
5. Better naming of example code.
6. Removed dependency on gflags in all the core examples covered
   in the tutorial.

Change-Id: Ibf3c7fe946fa2b4d22f8916a9366df267d34ca26

examples/powell.cc

@@ -46,7 +46,6 @@
 
 #include <vector>
 #include "ceres/ceres.h"
-#include "gflags/gflags.h"
 #include "glog/logging.h"
 
 using ceres::AutoDiffCostFunction;
@@ -55,8 +54,7 @@ using ceres::Problem;
 using ceres::Solver;
 using ceres::Solve;
 
-class F1 {
- public:
+struct F1 {
   template <typename T> bool operator()(const T* const x1,
                                         const T* const x2,
                                         T* residual) const {
@@ -66,8 +64,7 @@ class F1 {
   }
 };
 
-class F2 {
- public:
+struct F2 {
   template <typename T> bool operator()(const T* const x3,
                                         const T* const x4,
                                         T* residual) const {
@@ -77,8 +74,7 @@ class F2 {
   }
 };
 
-class F3 {
- public:
+struct F3 {
   template <typename T> bool operator()(const T* const x2,
                                         const T* const x4,
                                         T* residual) const {
@@ -88,8 +84,7 @@ class F3 {
   }
 };
 
-class F4 {
- public:
+struct F4 {
   template <typename T> bool operator()(const T* const x1,
                                         const T* const x4,
                                         T* residual) const {
@@ -100,7 +95,6 @@ class F4 {
 };
 
 int main(int argc, char** argv) {
-  google::ParseCommandLineFlags(&argc, &argv, true);
   google::InitGoogleLogging(argv[0]);
 
   double x1 =  3.0;