statistics: make timestamp stat mandatory.
statistics: when extracting quantiles from histogram stop on the last bin.

Still extracting 100%-quantile could lead to fail due to double rounding errors.
chrono: TICK time unit renamed to CYCLE.
chrono: TSC clock type renamed to INTERNAL.
chrono: tsc_clock renamed to internal_clock.
chrono: duration and time_point implementation moved out from header file.
chrono: time_unit comparator moved out from header file.
statistics: use default-constructed time_point as special value (marks uninitialized data).
statistics: when quantile_extractor fails return 0 instead of throwing exception.
benchmarks: don't use CYCLE time_unit manually.
chrono: duration helpers (e.g., chrono::milliseconds(), chrono::seconds()) added.
serialization: json serialization interface added (template loads and dumps functions).
serialization: json serialization specifications for statistics::data added.
chrono: time_unit and clock_type enum classes replaced with old-style enums.
chrono: INTERNAL and SYSTEM clock_type renamed to INTERNAL_CLOCK and SYSTEM_CLOCK.