graph/models/graph_model.coffee: Add file
graph/graph_model.coffee: Add spec
Edit graph/models/graph_model.coffee: グラフを表すクラスを実装

辺の追加と削除、辺の存在の確認、頂点数と辺の本数を確認する機能を追加
Merge branch 'feature/20140314-graph-class' into develop / #13