Implement close method

The connection_pool gem is implementing  automatic reaping of idle connections. In order to do this generically, clients need to provide a `close` method that the pool can call on reaped connections in order to close the network connection cleanly.

mysql2 and dalli already provide `close`. This PR adds this convention to redis-rb.

Author: mperham

lib/redis/client.rb

@@ -250,6 +250,7 @@ def connected?
     def disconnect
       connection.disconnect if connected?
     end
+    alias_method :close, :disconnect
 
     def reconnect
       disconnect