Fix handling of strings in SEO and file names

* URLify library updated to latest version (and customizations removed)
* added TextHelper::asciify() method
* TextHelper::sanitizeFileSystem() now does not removes words excluded
  for SEO
* FileHelper::sanitize() now correctly handles multibyte filenames; if the
  resulting filename is empty (for instance for Japanese file names) let's
  build the name from md5()
Fix to allow empty list of excluded words

Let's allow users to have an empty list of words to be excluded from URLs
fixing a test, fixing date helper
tweaks to text helper
Merge branch 'add-asciify' of https://github.com/mlocati/concrete5 into mlocati-add-asciify
adding file helper test
Merge branch 'master' of github.com:concrete5/concrete5