AutoDiff Code Generation - CMake Integration

This patch integrates the code generation module into the build
system. All depenendcies are tracked through CMake targets.
Modifying the cost functor will automatically trigger code
re-generation.

All this functionality is defined in the CMake function
ceres_generate_cost_function_implementation_for_functor
in CeresCodeGeneration.cmake. A hello world usage example
is included in examples/CMakeLists.txt.

Change-Id: I23b8b6698d1ea51cf3d788a47afcf39f8c5ce327