Model form page

This PR aims to add:
- the Model Page
- the Form for the entity Page

Notes:
you can generate the model and the form Page by running the following commands:
```
php app/console doctrine:generate:entity --entity=AcmeBlogBundle:Page   --format=annotation --fields="title:string(255) body:text"   --no-interaction
php app/console doctrine:generate:form AcmeBlogBundle:Page --no-interaction
```