Implement "crc16" client option for redis-cluster.

By default Predis chooses automatically which hash generator to use:
if a suitable version of ext-phpiredis is loaded (that is, a version
that implements the phpiredis_utils_crc16() function) it uses the new
Predis\Cluster\Hash\PhpiredisCRC16 generator, otherwise it falls back
to the usual pure-PHP Predis\Cluster\Hash\CRC16 generator.

tests/Predis/Configuration/Option/ClusterTest.php

@@ -102,12 +102,19 @@ class ClusterTest extends PredisTestCase
 
         $options = $this->getMock('Predis\Configuration\OptionsInterface');
         $options
-            ->expects($this->once())
+            ->expects($this->at(0))
             ->method('__get')
             ->with('connections')
             ->will($this->returnValue(
                 $this->getMock('Predis\Connection\FactoryInterface')
             ));
+        $options
+            ->expects($this->at(1))
+            ->method('__get')
+            ->with('crc16')
+            ->will($this->returnValue(
+                $this->getMock('Predis\Cluster\Hash\HashGeneratorInterface')
+            ));
 
         $this->assertInstanceOf('Closure', $initializer = $option->filter($options, 'redis'));
         $this->assertInstanceOf('Predis\Connection\Cluster\RedisCluster', $initializer($parameters = array()));