Pool (#16)

* add pool

* elastic workerpool

* del buf

* modify usage

* modify readme

broker/usage.go

@@ -0,0 +1,24 @@
+package broker
+
+var usageStr = `
+Usage: hmq [options]
+
+Broker Options:
+    -w,  --worker <number>            Worker num to process message, perfer (client num)/10. (default 1024)
+    -p,  --port <port>                Use port for clients (default: 1883)
+         --host <host>                Network host to listen on. (default "0.0.0.0")
+    -ws, --wsport <port>              Use port for websocket monitoring
+    -wsp,--wspath <path>              Use path for websocket monitoring
+    -c,  --config <file>              Configuration file
+
+Logging Options:
+    -d, --debug <bool>                Enable debugging output (default false)
+    -D                                Debug and trace
+
+Cluster Options:
+    -r,  --router  <rurl>             Router who maintenance cluster info
+    -cp, --clusterport <cluster-port> Cluster listen port for others
+
+Common Options:
+    -h, --help                        Show this message
+`