SCAN and variants

This commit introduces support for the Redis SCAN, HSCAN, SSCAN,
and ZSCAN commands.

In the case of HSCAN, SSCAN, and ZSCAN, we take a key and iterator
as required arguments, and for SCAN just an iterator.  Matching
the Redis commands, each variant can optionally take a pattern
to match against and a count value which hints at Redis how many
keys to return at a time.

When scanning keys or members (especially with a large keyspace when
searching for a pattern), Redis will sometimes return an empty result
of keys/members.  PHPRedis can be set up to abstract this from the
caller by setting:

$redis->setOption(Redis::OPT_SCAN, Redis::SCAN_RETRY);

Which instructs PHPRedis to keep retrying the scan until members are
returned OR the iteration completes (Redis returns to us a zero iterator).

By default this option is set to Redis::SCAN_NORETRY, meaning that
empty results are possible, requiring an explicit check for FALSE in the
scanning loop, like so:

```php
$it = NULL;
while(($arr_keys = $redis->scan($it, "*pattern*"))!==FALSE) {
	print_r($arr_keys);
}
```
Documentation fix
Another documentation fix :)
Merge branch 'feature/scan_commands' into develop