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'm not inspired either... what about "Gets info about the price from the
> external storage"?
>
Well, it's the Price External Storage. No need for a comment in that case.
I think VAT needs to be mentioned, as it is what external storage is
responsible for, in terms of business value.