Fix process kill on Mac OS X Mavericks

**The problem**: `guard-go` gets the `pid` of the `go run` command and not the pid of the running application. It sends the `KILL` signal to the `go run` process and exits, but the child process remains running.

**The solution**: change the application starting process to `go build` + `./binary`.

**Host:** Mac OS X Mavericks
```bash
$ go version
go version go1.3 darwin/amd64
```

**This pull request is based on #10.**