metrics: attribute moved from handystats::metrics to handystats namespace.
metrics_dump: common map of metrics splitted into two maps - metrics and attributes.
internal: 'handystats.internal.size' metric removed.
metrics_dump: metric object replaced with corresponding statistics object in metrics map.
json: metrics json writers removed.
rapidjson: rapidjson headers removed from public interface.
json: json writers removed.
utils: rapidjson_writer added.
config: configure() methods moved out from config structs.
config: ability to pass rapidjson::Value as config removed.
tests/CMake: add GTEST_TARGETS dependency only if Google Test is compiled from sources.
utils/rapidjson_writer: support rapidjson 'v1.0 Beta'.
statistics: insert new bin into histogram with move semantics.
statistics: reset histogram bin's timestamp if there's no data.
chrono: time_unit_from_string(), time_unit_as_string() added.
config: configuration format simplified:
- "statistics" -> "defaults"
- "core.enable" -> "enable"
- "tags" -> "stats"
- "metrics-dump.interval" -> "dump-interval"
examples: config example (config.json) and its description (config.md) added.
metrics_dump: attribute "handystats.dump_timestamp" replaced with attributes "handystats.system_timestamp" and "handystats.internal_timestamp".

Attribute "handystats.system_timestamp" represents unix timestamp in milliseconds to which current dump is related.
Attribute "handystats.internal_timestamp" represents unix timestamp in milliseconds to which metrics' state is related.
Almost always these timestamps will be the same except high income load, when core will not be able to process all messages timely.
statistics: full stats mask added (statistics::tag::all).
statistics: internal statistics' data moved out from statistics class declaration (pimpl idiom).
serialization: statistics data serialization to rapidjson added.
backends: file_logger added.
benchmarks/load: file_logger support added.
benchmarks/load: run_load.sh script calls load benchmark with file_logger options.
serialization: statistics deserialization from rapidjson added.
statistics: truncating mechanics for statistics data added.
statistics: compress_histogram function added.