disableLoopback in a can channel

canChannel.disableLoopback() allows hidding sent messages to other channels connected to the same can device.

see also https://github.com/sebi2k1/node-can/issues/3

```javascript
var can = require('can');

var listen       = can.createRawChannel("can0", true);
var sendStandard = can.createRawChannel("can0", true);
var sendHidden   = can.createRawChannel("can0", true);
sendHidden.disableLoopback();

// Log messages comming to can0 except those comming from sendHidden
listen.addListener("onMessage", function(msg) { console.log(msg); } );

listen.start();
sendStandard.start();
sendHidded.start();
```