Makes String#mb_chars on Ruby 1.9 return an instance of ActiveSupport::Multibyte::Chars to work around 1.9's lack of Unicode case folding.
Refactors class methods from ActiveSupport::Multibyte::Chars into new Unicode module, adding other related functionality for consistency.
[#4594 state:resolved]
Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>