Documentation update.

1. Complete restructuring of the documentation to account for
   GradientProblemSolver.
2. Update the version history to account for changes since 1.9.0.
3. Add links and document the various examples that ship with ceres.
4. Documentation for GradientProblem GradientProblemSolver.

Change-Id: If3a18f2850cbc98be1bc34435e9ea468785b8b27
19 files changed