Diverging from the v0.8 branch (current stable).

This commit marks the start of works for the next major release of
Predis which will bring various breaking changes needed to polish the
internal design making the library even more flexible to use or extend
and, more importantly, almost stable in terms of API.

The Redis commands API exposed by Predis\Client is not going to change
much if not at all which is a good news. The most immediate changes
affecting developers will involve the renaming of a few namespaces and
classes, the removal of some previously deprecated classes and methods
and some tweaks to the current abstractions.

Right now the plan is to have a fast paced development to release this
version as soon as possible, ideally a few weeks later than Redis 2.8,
then wait to see the final definition of redis-cluster so that we can
tweak our code if needed and finally hit the v1.0.0 milestone with as
few changes as possible. Furthermore, v1.x will most likely be the
last version of Predis supporting PHP 5.3 as we will start migrating
to PHP 5.4 (or even 5.5) with v2.x, which is not going to happen soon
anyway.
Reimplement from scratch client configuration.

This commit is a complete rewrite of the classes previously contained
in the Predis\Option namespace aimed at lowering the initialization
overhead while bringing in more consistency. The overall idea is still
the same with a mini DI container, Predis\Configuration\Options, which
carries options with values that can be initialized lazily.

The first difference with our previous implementation is that now even
user-defined options can be initialized lazily, everything needed is
an object responding to the __invoke() magic method such as a closure.
Other kind of callable arguments (strings, arrays) will be treated as
plain values. The only drawback is that we cannot pass any instance of
classes implementing __invoke() as an option value, but considered the
limited scope of our use case we can say it's more of an acceptable
compromise. Callbacks used for lazy initialization will receive two
arguments upon invokation:

  - The current instance of Predis\Configuration\Option ($options)
  - A string containing the name of the option ($option)

This is an example in actual code:

  $options = new Predis\Configuration\Options([
    'exceptions' => true,
    'profile' => '2.8',
    'distributor' => function () {
      return new Predis\Cluster\Distribution\KetamaPureRing();
    },
    'cluster' => function ($options) {
      $distr    = $options->distributor;
      $strategy = new Predis\Cluster\PredisClusterHashStrategy($distr);
      $cluster  = new Predis\Connection\PredisCluster();

      return $cluster;
    },
    'connections' => function ($options, $option) {
      $factory = $options->getDefault($option);
      $factory->define('tcp', 'Predis\Connection\PhpiredisConnection');

      return $factory;
    },
  ]);

As you can see there's very little difference compared to before in
the actual usage as most changes are under the hood. Some options such
as "exceptions" and "replication" can now correctly parse bool values
from strings (so the string "false" is not evaluated as boolean true).

While options were initially conceived to configure the client and its
behavior, the concept has matured and it's perfectly fine to consider
the use of Predis\Configuration\Options to propagate configurations to
inner parts of the library.
Merge branch 'v0.9/client-options'
Remove previously deprecated Predis\Helpers class.