[4.2] Add Non Latin Character Support For Str::slug

[reshadman]

Add support for UTF-8 encoded slugging. In case of slugging persian characters (like آ, ب, ژ) converting encoding to ASCII causes an empty string.

// سلام-دنیا
Str::lightSlug('سلام دنیا'); 

//empty string 
Str::slug('سلام دنیا');

[GrahamCampbell]

The tests are failing @bigsinoos. Also, this is best sent to 5.0 because of the change in the method signature. Also, you'll need to fix the docblock for the extra method you added.

[lucasmichot]

This has been told many times in PR, but maybe replacing the Str helpers with https://github.com/danielstjules/Stringy would be a good idea

[GrahamCampbell]

@lucasmichot, @taylorotwell already rejected my pull to move us over to stringy.

@taylorotwell Would you reconsider for laravel 5.0?