Updated documentation

I've also marked hexa as deprecated and moved `countryCode` to
its alphabetical position.
New Xdigit rule based on the previous Hexa rule

This is a really ugly copy'n'paste.  There are two problems to
solve: how to make 'hexa' deprecated and how to make its use fall-
back to xdigits.

The simplest solution would be to put a if in Validator::buildRule,
but it's too hackish for my taste.
Small merge to amend a commit that I've alread pushed