Implement VAT support

Follow up of https://github.com/ezsystems/EzPriceBundle/pull/10

**WIP** . Maybe a starting point for the Storage Part? Some things are already done
Actually we are only caring about get data from the storage and present it in the template
- [x] Add services definition
- [x] Create External Storage Class 
- [x] Create Storage Gateway Class
- [x] Get vat percentage to apply from the legacy storage
- [x] Modify Value constructor and test to accept arrays as input
- [ ] Create a new class for doing the calculations needed to obtain the price with vat excluded and with vat included
- [ ] Present that info in the template
We need a class comment. Suggestion:
```php
/**
 * Price FieldType external storage handler.
 * Handles the VAT rate.
```

Not inspired...