add log to file support

example-config.json

@@ -7,7 +7,7 @@
             "contact_points":"127.0.0.1",
             "port":9042,
             "keyspace":"clio",
-	        "replication_factor":1,
+            "replication_factor":1,
             "table_prefix":"",
             "max_requests_outstanding":25000,
             "threads":8
@@ -36,6 +36,7 @@
         "port":8080
     },
     "log_level":"debug",
+    "log_file":"./clio.log",
     "online_delete":0,
     "extractor_threads":8,
     "read_only":false