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