Added ability to send extra information

* If the instance has `extras` assigned send object instead and
`logs` as a property.
* ~~remove cid query string as this should be done through `extras`~~
* Keep cid query string *only* when no `extras` are provided. 

I think cid format should be up to the user and with this PR they can do:

```js
var logger = new Minilog.backends.jQuery();
logger.extras.cid = 1234567;
```

That will post:

```js
{
   cid: 1234567,
   logs: [...]
}
```

If no extras are specified it will POST in the same format as it does now.

mixu/minilog#17