2.2.0: Optional features

• Made Optional's constructor public and allow new Optional(null) to
  represent an optional with a present null value
  (@thekid)
• Changed Optional::of() to return an empty optional if null is passed
  (@thekid)
• Added whenNull() method to util.data.Optional to allow easy chaining
  (@thekid)
• Added filter(), map() and orUse() implementations to util.data.Optional
  See pull request #21
  (@thekid)