feat($http): setting JSONP callback name

Allow to define a name to angular.callbacks function. It would be useful to caching (varnish, etc).

Ex:
```javascript
$http.jsonp("http://example.com?callback=JSON_CALLBACK(myFunction)").success(...)
```

Will always request
```
http://example.com?callback=angular.callbacks.myFunction
```

I would reword it more like this:

```js
var base = (/JSON_CALLBACK\((.*?)\)/).exec(url);
base = base ? base[1] : '';
callbackId = base + ('_' + (callbacks.counter++).toString(36));
```

This way we use the nam