Adds unicode support via unicode gem

In regular Ruby `String#upcase` doesn't take into account the special UTF-8 character in some languages. E.g. in Danish, we have three additional letters æ, ø, and å, which in uppercase is Æ, Ø, and Å.

The following aren't thus working:

```ruby
str = "æøå"
str.upcase # => "æøå"
# but should have been "ÆØÅ"
```

The `unicode` gem solves this problem (Also, in Rails we have `ActiveSupport::Multibyte::Chars`)

With this, we can instead do

```ruby
str = "æøå"
Unicode.upcase(str) # => "ÆØÅ"
# this is now correct
```
Why `String(..)` instead of `to_s`?