Support keyspace notifications #23

Hey!

As an extension of a StackOverflow Q&A I've published today (http://stackoverflow.com/questions/23180765/redis-keyspace-notifications-with-stackexchange-redis), I'm adding here the issue in order to request keyspace notifications.

Thanks in advance, either if it gets implemented or not 👍

EDIT

According to the StackOverflow Q&A linked above, we arrived to the conclusion that keyspace notifications are just pubsub channels, and it would be nice that StackExchange.Redis would implement helpers, since keyspace event channel names would require to work with strings (not that elegant).

These helpers would look something like:

    IDatabase.KeySpaceEvents.OnSet("users:matias:messages", (channel, value) => { });

Another good point would be specifying which command to listen:

    IDatabase.KeySpaceEvents.OnSet("users:matias:messages", (channel, value) => { }).When(RedisCommand.SAdd);

It's just a suggestion ;)