Added chrono-style clock

This is a std::chrono style clock that can be used with std::chrono calculations.

It appears that there is no guarantee in the C++ standard about the epoch, but it is 1970-01-01 on all our current targets. If it deviates, then I have added the MAIDSAFE_CLOCK_EPOCH_OFFSET_IN_DAYS macro to adjust the epoch. This solution has no performance impact on our current targets (and a minimal impact on future targets that may deviate.)