file route uses __dirname

If you try to use `echo/get/?file=assets/checknet.txt` from yeti (for example), it uses __dirname so the end file is 
`/Users/nicols/git/yui/yeti/node_modules/echoecho/lib/assets/checknet.txt`

Ideally it should use a more relevant directory. I'm not sure whether this should be the process cwd(), or whether it should take an override somewhere...

```
    this.echoecho = new EchoEcho({
        all: true
    });
    if (this.echoecho.handle(path)) {
        this.echoecho.setRoot(pathRoot);
        this.echoecho.serve(server.req, server.res);
    }
```