VAT management for EU

[cbellone]

The following doesn't apply to free of charge events/reservations.

As per http://europa.eu/youreurope/business/vat-customs/buy-sell/invoices/index_en.htm the EU VAT application rules are somewhat unclear about what exactly is a service.
Therefore we should let the organizer choose whether or not to apply VAT to business customer and for which countries.

requirements:

• the organizer must specify the country where he/she runs the business (either upon creation or afterwards)
• the organizer can enable this behavior (at organization/event level) and define the countries where the VAT does not apply. We can provide two templates for that: all foreign countries, all EU countries
• the end user must clearly state that he/she is a company, by clicking on "I want an invoice" (new) before paying and then by entering his/her VAT nr. (new)

whether or not the VAT number should be also validated against some webservice is unclear at the moment.

[cbellone]

Done.
Validation has been implemented using https://github.com/stephanj/ServerlessEUVATChecker