Filename normalize rules & behavior changed

I think that `_normalizeFilenames` should be turned on by default because user shouldn't create invalid file and directory names.

I changed behavior of normalizer like this:

* `uploader::checkFilename()` doesn't check for regexp -> when we change normalizing function, the previous implementation of `checkFilename` had own rules and caused problems even when filename had right format given by normalizing funcion. I think it's normalizer response.
* `file::normalizeFilename()` uses more robust and better implementation derived from [Nette Framework](http://nette.org) 
  * it translates all names to URL-friendly format, the previous implementation failed eg. on czech diacritics
  * eg. "nová složka" from user's input will be saved now as "nova-slozka" (URL-friendly)