use nullopt
fixes for g++-4.8 and clang 3.3 errors
new & improved synchronize: const support, operator (), sync(), maybe_sync(), locked views
new & improved metrics: group, group_map, simpler interface
use emplace_t from optional<>
Copy protect synchronized<>.
Merge pull request #58 from chipdude/master

second try at sync and metrics remodeling
Merge branch 'master' into task_evolution