Implement PHP iterator based on the SCAN command (Redis 2.8).

This iterator allows to perform full iterations over the keyspace of a Redis
instance by wrapping the incremental nature of SCAN using an abstraction that
fits perfectly in userland code:

  $client = new Predis\Client('tcp://127.0.0.1', ['profile' => '2.8']);
  $iterator = new Predis\Iterator\Scan\KeyspaceIterator($client);

  foreach ($iterator as $key) {
      echo $key . PHP_EOL;
  }

Memory consumption during an iteration is limited because elements are fetched
incrementally, on the other hand SCAN gives limited guarantees about returned
elements since the underlying collection (the keyspace in this case) can change
during the whole iteration process. The most immediate drawback is that the same
element may be returned multiple times.

See http://redis.io/commands/scan to fully understand the inner workings,
and particularly the "Scan guarantees" paragraph.
Extract base class for iterators based on the SCAN family of commands.

The iterators based on SCAN, SSCAN, ZSCAN and HSCAN will extend this abstract
class to share most of the logic which is common for all of the Redis commands
in the SCAN family.
Implement PHP iterator based on the SSCAN command (Redis 2.8).

This iterator allows to perform full iterations over the members of a set
by wrapping the incremental nature of SSCAN just like we did for SCAN:

  $client = new Predis\Client('tcp://127.0.0.1', ['profile' => '2.8']);
  $iterator = new Predis\Iterator\Scan\SetIterator($client, "set_key");

  foreach ($iterator as $member) {
      echo $member . PHP_EOL;
  }

Being SSCAN closely related to SCAN, it is subject to the same behaviour,
see http://redis.io/commands/scan for reference.

Meh
Implement PHP iterator based on the ZSCAN command (Redis 2.8).

This iterator allows to perform full iterations over the members of a sorted
set by wrapping the incremental nature of ZSCAN just like we did for SCAN:

  $client = new Predis\Client('tcp://127.0.0.1', ['profile' => '2.8']);
  $iterator = new Predis\Iterator\Scan\SortedSetIterator($client, "zset_key");

  foreach ($iterator as $member => $rank) {
      echo "$rank => $member" . PHP_EOL;
  }

Being ZSCAN closely related to SCAN, it is subject to the same behaviour,
see http://redis.io/commands/scan for reference.

This iterator implementation returns the member as key and the rank as value
since the rank is a float value which would be truncated when transforming
the iteration to an array (e.g. using iterator_to_array()). Luckily PHP
preserves the insertion order for named arrays members still result sorted.
Implement PHP iterator based on the HSCAN command (Redis 2.8).

This iterator allows to perform full iterations over fields and values of a
hash by wrapping the incremental nature of HSCAN just like we did for SCAN:

  $client = new Predis\Client('tcp://127.0.0.1', ['profile' => '2.8']);
  $iterator = new Predis\Iterator\Scan\HashIterator($client, "hash_key");

  foreach ($iterator as $field => $value) {
      echo "$field => $value" . PHP_EOL;
  }

Being HSCAN closely related to SCAN, it is subject to the same behaviour,
see http://redis.io/commands/scan for reference.
Update CHANGELOG and README.
Add example of iterators based on the various SCAN commands.
[ŧests] Add tests for new SCAN, SSCAN, ZSCAN and HSCAN iterators.

Right now we don't have integration tests since the returned values from
Redis may not always be predictable, but the current tests should suffice.
Fix SCAN-based iterators following failing tests.
[tests] Fix execution order of iterator methods.

We were testing iterators following a broken assumption about the
order in which the Zend engine executes the various methods during
the iteration.
[tests] Add more tests for SCAN-based iterators.
Move Redis collections iterators in a different namespace.

The base iterator class is now known as RedisCollectionIterator and
we've also renamed a few methods to make their naming more generic.
Rename for the last time all the iterator classes.

We are experimenting with a new approach at naming classes using less
redundant names by leveraging the containing namespace. The PHP "use"
directive is not limited to class names but can be used to import the
whole namespace, which means you can do something like this:

  use Predis\Collection\Iterator;
  // ...
  foreach (new Iterator\Keyspace($client) as $key) {
  	// ...
  }

Alternatively you can always rely on "use ... as ..." to import one of
the classes by giving it a more meaningful name in the context of the
root namespace:

  use Predis\Collection\Iterator\Keyspace as KeyspaceIterator;
  // ...
  foreach (new KeyspaceIterator($client) as $key) {
  	// ...
  }

In this specific case we chose to apply the -Key postfix to classes
iterating Redis keys to be more explicit about the fact that those
iterators does not work on local in-memory collections, but fetch
items from a key stored on a remote Redis server.
Mostly useless tweak for iterators example.
Tweak phpdocs for the abstract cursor-based iterator.
Merge branch 'feature/scan-iterators' into v0.8
[tests] Do not check exception message for connection timeouts.

Given the IP address we use for this test (the standard IP assigned by
operating systems when unable to get one lease from a DHCP server), on
certain kernels or network stacks we can get different messages such
as "No route to host". Since we just need to make sure that our test
does not take more than 1 seconds abiding by the sub-second timeout,
we can just live with that.