Implement sorting using twitter_cldr

[aried3r]

This is a draft PR since it does not necessarily overlap with the goals of this gem.

Using twitter_cldr we can solve #20 and partly #18. #20, because we can now pass a German collator and the result will be correct.
Case insensitive sorting is only partly solved as can be seen in the new specs. Lower case letters are sorted before upper case letters, however this might give surprising results, depending on your use case. As you can see, "öl3" is not after "Öl2", because aforementioned rule.

Before implementing self.sort_with_collator I tried using the implementation using a collator as the general case and all specs passed, so it seems that it could potentially be extended to have all methods take an optional collator. I didn't go that far for this PR since it might collide with the goals of this gem as it introduces an optional gem dependency.

Maybe it can also serve others as a pointer on how to solve similar issues.