String class rewrite

As was described in issue #816 current Str class has some serious problems with transliteration of numerous characters during `slug()` and `ascii()` calls. This PR moves Str class into it's own "package" and brings back good old character transliteration map.

Besides the fact that now Str is actually an instantiated class, it also has some extra features:

* **Remove List** - an array of words (defined in config file) which will be deleted on `slug()` call. Very useful when you need to remove conjunctions and such words which don't add content to the string

* **Language Priority** - uses locale (and `setLanguage()` method) to determine which language has priority during transliteration. Very useful for characters like `ö` which is transliterated to `o` from Latin and to `oe` from German

Tests included.