Add Redis store implementation

[rbcervilla]

Added Redis store implementation

1. Redis store implementation with go-redis as Redis client. This enables use Redis on single mode, Sentinel and Cluster.

[elithrar]

I took a quick look over your store implementation, and a couple of pieces of feedback:

• Make the README example a self-contained, runnable example
• You should provide an option to set a store-wide MaxAge instead of defaulting to 86400*30 (or requiring a per-session override)
• Make sure to apply the MaxAge to all Codecs - see Third Party Stores Not Applying Options to Codecs correctly. #48 (comment)

[rbcervilla]

• Make the README example a self-contained, runnable example

I have included a runnable example

• You should provide an option to set a store-wide MaxAge instead of defaulting to 86400*30 (or requiring a per-session override)

It's provided with method Options. Example:

    store.Options(sessions.Options{
        Path:   "/path",
        Domain: "example.com",
        MaxAge: 86400 * 60,
    })

• Make sure to apply the MaxAge to all Codecs - see #48 (comment)

This is not needed because the implementation doesn't use securecookie.

[rbcervilla]

@elithrar Do you think it is possible to add the implementation to the documentation?

[elithrar]

Thanks for this!