Retwis ported to modern Redis data types.

The example (and related documentation at redis.io) was not updated for
5 years. Redis had no sorted sets and hashes when the original code was
written (!). An update was really needed.

We also use a modern PHP client now: Predis. A copy is shipped within
this repository to make life easier to newcomers trying Redis for the
first time via this example.

README.md

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+Retwis is the Redis Hello World. A minimal Twitter-style social network clone
+written using Redis and PHP. The source code is designed to be very simple and
+at the same time to show different Redis data structures.
+
+You can find a tutorial explaining step by step how the example was created
+in the [Twitter clone tutorial of the Redis documentation](http://redis.io/topics/twitter-clone).
+
+This code was written back in 2009 when Redis was initially published, in order
+to expose interested developers to the Redis concepts and data types.
+Later, in May 2014, it was reworked in order to update it to *modern Redis*,
+since when the example was introduced Redis lacked sorted sets, hashes,
+and many other features.
+
+The following is a description of the data layout.
+
+Users
+---
+
+User IDs are generated sequencially via increments:
+
+    INCR next_user_id => 1000
+
+(We'll use the example user ID 1000 in the next examples)
+
+Users are stored into Redis hashes, the key name of the hash representing a
+given user is `user:1000`. Every hash has the following fields:
+
+    username (the username of the user)
+    password (password of course)
+    auth 9458sd893448dfd
+
+There are additional keys for following, followers, and posts:
+
+    following:1000 (sorted set of user ids)
+    followers:1000 (sorted set of user ids)
+    posts:1000 (list of posts ids)
+
+In order to reverse lookup the user ID from the authentication toke or
+username we have the following two additional keys:
+
+    The key `auths` is an hash mapping auth tokens to user IDs.
+    Example of field and value: 9458sd893448dfd => 1000
+
+    The key `users` is an hash mapping usernames to user IDs.
+    Example of field and value: antirez => 1000
+
+Additionally we take a sorted set with usernames indexed by registration
+time (the score is the unix time the user joined), so that we can populate
+our "latest users" view. The key is called `users_by_time`.
+
+Posts
+---
+
+Posts also have sequencial IDs, generated by incrementing the following key:
+
+    INCR next_post_id => 134
+
+Every post is stored into an hash named `post:134` with the following
+fields:
+
+    user_id
+    time
+    body
+
+Timeline
+---
+
+Just a list of post IDs in the `timeline` key.