fix(echo): remove echo, use bytes.NewBufferString, fixes #3

@philips took a swing at this. The only problem is that I keep getting transient failures. Do you have any idea what I might be doing wrong?

```
Alexs-MacBook-Air-3:nova-agent-watcher polvi$ go test -v .
=== RUN TestSSH
--- PASS: TestSSH (0.00 seconds)
=== RUN TestNet
--- PASS: TestNet (0.10 seconds)
=== RUN TestHostname
--- PASS: TestHostname (0.00 seconds)
=== RUN TestShadow
--- PASS: TestShadow (0.00 seconds)
PASS
ok  	github.com/coreos/nova-agent-watcher	0.144s
Alexs-MacBook-Air-3:nova-agent-watcher polvi$ go test -v .
=== RUN TestSSH
--- PASS: TestSSH (0.00 seconds)
=== RUN TestNet
--- PASS: TestNet (0.09 seconds)
=== RUN TestHostname
--- PASS: TestHostname (0.00 seconds)
=== RUN TestShadow
--- PASS: TestShadow (0.00 seconds)
PASS
ok  	github.com/coreos/nova-agent-watcher	0.131s
Alexs-MacBook-Air-3:nova-agent-watcher polvi$ go test -v .
=== RUN TestSSH
--- PASS: TestSSH (0.00 seconds)
=== RUN TestNet
2014/03/16 21:40:21 error: script wait failed write |1: broken pipe
--- FAIL: TestNet (0.01 seconds)
	main_test.go:80: write |1: broken pipe
=== RUN TestHostname
--- PASS: TestHostname (0.00 seconds)
=== RUN TestShadow
--- PASS: TestShadow (0.00 seconds)
FAIL
exit status 1
FAIL	github.com/coreos/nova-agent-watcher	0.047s
```