Move serializer.xml loading

The usage of serializer.xml services can be not restrictive to REST API only.
In some cases it is usefull to serialize objects in an internal workflow, so you don't have to load FOSRestBundle nor NelmioApiDocBundle.

For example, in the Sonata commerce bundle, when you move a product from Basket to Order, each product items are serialized but not used in a REST interface.

The same modification can also be done in the following bundles:
- sonata-project\ecommerce\src\CustomerBundle\DependencyInjection\SonataCustomerExtension.php
- sonata-project\ecommerce\src\InvoiceBundle\DependencyInjection\SonataInvoiceExtension.php
- sonata-project\ecommerce\src\OrderBundle\DependencyInjection\SonataOrderExtension.php
- sonata-project\ecommerce\src\ProductBundle\DependencyInjection\SonataProductExtension.php
- sonata-project\news-bundle\DependencyInjection\SonataNewsExtension.php
- sonata-project\page-bundle\DependencyInjection\SonataPageExtension.php
- sonata-project\user-bundle\DependencyInjection\SonataUserExtension.php
Merge pull request #510 from loicolas/patch-1

Move serializer.xml loading
Add gallery/media relation APIs
Merge pull request #512 from eko/api

Add gallery/media relation APIs
Merge remote-tracking branch 'refs/remotes/sonata-project/master'