GraphModelのテストに、既に存在する辺を追加するテストを追加
Merge remote-tracking branch 'github/feature/20140315-add-test-graphmodel' into feature/20140317-add-test-graph-model
Merge branch 'feature/20140315-add-test-graphmodel' into next/feature/20140315-add-test-graphmodel
graph_model: コンストラクタのオプションに関するテストを追加
Merge branch 'feature/20140315-add-test-graphmodel' into develop / #26