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
I don't think we need the comments here. With properly named variables, it will be understandable. What about this:

```php
$price['price'] = $priceRow['data_float'];

list( $isVatIncluded, $vatTypeId ) = explode( ',', $priceRow['data_text'] );
$price['is_vat_included'] = ( $isVatIncluded == 1 );
$price['vat_percentage'] = $this->getVatPercentage( $vatTypeId );
```

I think it's perfectly readable.