New commands: PFADD, PFCOUNT, PFMERGE (Redis 2.8.9).

Backported from the master branch.

Many thanks to @rubensayshi for his initial commits on this in #163,
but I had to start from scratch for the master branch first.

tests/Predis/Profile/ServerVersion28Test.php

@@ -177,6 +177,9 @@ class ServerVersion28Test extends PredisProfileTestCase
             136 => 'sscan',
             137 => 'zscan',
             138 => 'hscan',
+            139 => 'pfadd',
+            140 => 'pfcount',
+            141 => 'pfmerge',
         );
     }
 }