Pending changes from the last PR

I guess this PR closes all the four major issues from the last one:

  * Single line statements without braces, please.
  * Ensure 100% test coverage
  * Fix duplication checking for empty input and comparing change.- pending cleanInput check
  * Make sure that allow empty stays or goes - pending all allow blank

I've also made `v::hexa()` deprecated in favour of `v::xdigits()`.  When hexa is used, it'll trigger an error of level E_USER_DEPRECATED.  This error level was introduced in PHP 5.3.  I don't remember seeing anywhere what's the minimum version of PHP necessary to use Validation, but if it's 5.2, we'll need to use other error level.

I've also made some changes on documentation.