Benchmark for messenger

bench.txt

$ go test -v -run=Bench* -bench=. -cpu=4 -send-routines=4
PASS
BenchmarkMessengerSendSmallMessage-4	   50000	     58833 ns/op
BenchmarkMessengerSendMediumMessage-4	   50000	     58367 ns/op
BenchmarkMessengerSendBigMessage-4	   50000	     56553 ns/op
BenchmarkMessengerSendLargeMessage-4	   50000	     56711 ns/op
BenchmarkMessengerSendMixedMessage-4	   50000	     61250 ns/op
BenchmarkMessengerSendRecvSmallMessage-4	   50000	     46408 ns/op
BenchmarkMessengerSendRecvMediumMessage-4	   50000	     44517 ns/op
BenchmarkMessengerSendRecvBigMessage-4	   50000	     45937 ns/op
BenchmarkMessengerSendRecvLargeMessage-4	   50000	     47275 ns/op
BenchmarkMessengerSendRecvMixedMessage-4	   50000	     43715 ns/op
ok  	github.com/yifan-gu/go-mesos/messenger	112.142s

environment:
Linux yifan-laptop 3.13.0-32-generic #57-Ubuntu SMP Tue Jul 15 03:51:08 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
CPU: Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz
MEM: 4G DDR3 1600MHz