* Included in CMakeLists option to enable/disable use of boost (and check if it is found on system).
* Modified 'error.h' to use a custom 'format' (using variadic template) instead of boost::format.
* Created 'optional.h' to wrap 'boost/optional' (when using boost) or using custom optional (without boost).
    * Changed 'repo.cpp' to use it
* Modified 'diff.cpp' to allow alternate between 'boost::flatmap'(when available) or 'std::unordered_map'
* Modified examples (log.cpp and rev-parse.cpp) to align with the changes.

* Changed custom `optional` implementation to avoid dynamic allocation
* Changed `format` implementation to use `std::snprintf` instead of stream
    * Using `boost::format` if using boost
* Moved 'internal' headers to specific folder
* Minor changes (in `examples/log.cpp` and `src/repo.cpp`) to adapt to new implementations.