metrics: convenient extraction methods for gauge internal statistics have been added.
core: don't return from finalize() if enabled flag is unset.
system_stats: system internal statistics are declared within its own namespace handystats::system_stats now.
system_stats: initialize() and finalize() methods have been added.
internal_metrics: internal_timer constructors have been fixed.
configuration: external handystats configuration has been added.
internal: check internal_timer on timeout before processing event message.

Configuration should be passed in json format (via config file, rapidjson::Value or const char*):
{
   "handystats": {
       "gauge": {
           "moving-average-alpha": <double value>,
           "moving-interval": <value in msec>
       },
       "timer": {
          "idle-timeout": <value in msec>
       },
       "json-dump": {
           "interval": <value in msec>
       },
       "message-queue": {
          "sleep_on_empty": [<first sleep interval in usec>, <second sleep interval in usec>, ...]
       }
   }
}
json: write_to_json_... functions have been added for system statistics. System statistics' json_dump format has been changed.

system statistics are now in the following form in json_dump:
{
    ...
    "__system-stats": {
        "message-queue-size": {
            ...
        },
        "internal-metrics-size": {
            ...
        },
        "message-process-time": {
            ...
        },
        "message-pop-time": {
            ...
        }
    }
}