KVSの読み書きを行うライブラリを作成する

[emiksk]

Created from AI0387 of minute 440.

• KVSの読み書きを行うライブラリを作成する．

[emiksk]

I addressed this issue in #6(in particular 770a83e and b8a72b8).
I implemented DataStore class which has abstract methods such as load, store and delete.
RedisStore class inherit DataStore, which is implemented concretely above methods.
When create the RedisStore object, write like DataStore.create(:redis) rather than RedisStore.new.
If necessary, other subclass can be implemented such as MongoStore.
This implementation is sharing in camome(mentioned camome/#47), but not library.

[emiksk]

PR #13 でDataStoreクラスをHashと同じようなインタフェースで扱えるように変更した．