core: stat_values added to call_tree

Stat values allow to store key-values pairs in call tree. They are
implemented as map<string, boost::variant>, where boost::variant is
stuffed with several default types.
api, stats: public api for adding stats to tree added
examples: example of using stats added
aggregators: filter aggregator added. You can filter aggregated trees
with explicitly set filter function.
examples: filter_aggregator example added
tests: basic stats tests added