Use Predis\Response\Status to identify all kinds of status responses.

Status response objects are needed mostly to make it possible from the
client perspective to differentiate a status response with the payload
"OK" from a normale bulk reply containing "OK".

The biggest change is for commands returning +OK responses: these were
previously translated to TRUE (bool value), but they are now returned
as instances of Predis\Response\Status. Just to illustrate an example
of the possibilities with this change we will use SET since it is the
most widely used command returning +OK:

  $response = $client->set('foo', 'bar');

  echo $response;         // 'OK'
  $response == 'OK';      // TRUE
  isset($response->ok);   // TRUE
  $response == true;      // TRUE
  $response === true;     // FALSE
  $response instanceof Predis\Response\ObjectInterface;     // TRUE
  $response instanceof Predis\Response\Status;              // TRUE

For those checking responses returned by commands such as SET or PONG,
the breaking change basically lies in the usage of strict comparison:
doing $response === true will now evaluate to FALSE instead of TRUE.

By default Predis caches common status responses such as OK or QUEUED
to lower the memory usage when using pipelines or transactions.
Merge branch 'v0.9/response-status'
Rename base response interface to Predis\Response\ResponseInterface.
Apply code styling fixes.

There is actually no need for such indentations.