Add UUID() without param to generate random UUID

Uses the  method to generate a valid v4 UUID.

Usage:
   UUID() # or Youyouaidi::UUID.new()
   # => #<Youyouaidi::UUID:0x00000102201b80 @converter=Youyouaidi::Converter,
   #                                        @uuid='27f8bc29-be8e-4dc7-ab30-0295b2a5e902'>
Add more restriction to .valid?

UUID needs to have either a '8', '9', 'a', or 'b' as first character of
the fourth group.

See:
https://en.wikipedia.org/wiki/Universally_unique_identifier#Variants_and_versions