Start development of new major version.

This new version will require Predis v0.8 and will break compatibility.
Rework the whole provider for v0.4.0 (now requires Predis v0.8).

Nothing changes for single-client configurations, but there is
a BC for multiple-clients configurations since the addition of
the `boot()` mechanism in Silex essentially made impossible to
keep this feature alive without artificially boot()-ing the
provider (which was no good).

Multiple clients are now supported only when using the new
`Predis\Silex\MultiPredisServiceProvider` but the way clients
are configured remained the same.

Clients in such configuration are accessible in the following
manner:

  $client = $app['predis'][$alias];

It is no more possible to configure a default client responding
to `$app['predis']` due to the nature of the above change.
Add missing phpdoc.
Merge predis.default_parameters even when parameters are URI strings.

Too bad Predis\Connection\ConnectionParameters::parseURI() is private
forcing us to copy and paste its implementation here.
Remove useless stuff.
Improve multiple clients example to use default parameters and options.
Add example of how to use Redis for handling PHP sessions.

Predis v0.8.2 is required to use Predis\Session\SessionHandler.
Use predis.clients_container to customize client container initializer.
Define a default client in a multiple-clients configuration.

This brings back 90% of what was possible to do in previous versions
of this service provider, with only two caveats:

  - Accessing the default client has obviously more overhead compared
    to getting the client explicitly and invoking a method against it.

  - Predis\Client::keys() is overridden by Pimple::keys().

This is merely a shortcut to access the most used client instance among
multiple ones, but you can define your own clients container to return
a plain instance of Pimple with "predis.clients_container" if you dislike
it so much.
Merge branch 'v0.4-wip'