Moved namespace definition to dump method

This commit allows to define custom javascript namespace:

```javascript
App::make('JSLocalizeDumper')->dump() // 'window.app' by default
App::make('JSLocalizeDumper')->dump('mynamespace') // 'window.mynamespace' 
```
Usecase: I output Javascript variables along with HTML data in my ajax responses. 
Without this commit, previous window.app object gets overwritten on each ajax responses.