Improved storage capabilities

[Kariton]

because the current implementation of storage.read() or storage.save() do not work well when they run simultaneously i moved the _redis from storage class to redis.
now the entire redis class is available within the storage class.
furthermore storage.delete() is now a thing.

i have a command that fetches content from a website. this takes time.
to make sure a user never receives the same post twice i used to:

1. storage.read()
2. do stuff
3. storage_result.append(new_id)
4. storage.save()

if the user runs the same command twice in a row it might happen that both coroutines read the same data and only the last one is actually saved.

now this is possible:

if c.bot.storage.redis is not None: # 'None' is default if storage is not redis
    c.bot.storage.redis.smembers(storage_key) # use redis.Redis().smembers() - list the set of stored data
    c.bot.storage.redis.sadd(storage_key, item["id"]) # use redis.Redis().sadd() - add item to the end of the existing data

ill close #35 in favor of this PR.
does not: fixes #29

typo: does not fix 29

[Kariton]

i was ahead of my own commits... fixed