Removed duplicated rule Xdigits (there is Hexa already)
Removal of braces on single-line statements

I've only removed braces from the files I'd changed/created on my
last pull request.  Notice, however, that there are other files
where this still happens.
Removal of $acceptEmptyString

Notice that this changes the behaviour of some rules (of Digits in
special).  That's why some lines from the tests appear in the diff,
they were removed from the invalid data provider and where put into
the valid data provider.
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.
Mark Hexa as deprecated

I've also changed its tests, setting an error handler that does
nothing.  Maybe we should check if E_USER_DEPRECATED is really
being triggered, but I'm not sure on how to do it.
Removed unnecessary check in AbstractCharGroup
Refactored AbstractCharGroup into AbstractRegexRule

It's now an actual abstract class.
Made OneOfTest use Xdigits instead of Hexa
Merge pull request #110 from ramaciotti/develop

Pending changes from the last PR