Merge core::stat::mixable_stat to core::stat::stat
Remove core::stat::mixable_stat
Move mixable_stat to core/stat
Add register_mixables_to_holder to stat.

This change contain a workaround to implement this function, because stat is not split to algorithm and model.
It uses itself via shared_from_this as its model.
Add an additional TODO comment
Use canonical interface and delegating_mixable for stat and mixable_stat
Move mixable_graph to core/graph and refactor graph
Make stat::register_mixables_to_holder const
Move mixable definition to bottom of the header
Remove type for mixed value: use diff type instead
Change diff type of graph
Merge branch 'develop' into feature/refactor_stat_and_graph
Fix cpplint errors
Merge pull request #450 from jubatus/feature/refactor_stat_and_graph

Feature/refactor stat and graph for #412