Add Optional type and test to collection library.
Added comments. Added to top level doc. Renamed get => value. Changed "transform" signature. Changed toString.
Add ifPresent and ifAbent methods to Optional.
Make constructors const where possible.
Remove optional callbacks from isPresent and isAbsent.
Remove Optional.asSet.
Stop referring to _value in comments where it's not visible.
Don't mention private values in docs.
Allow transformer to change type.
Moved Optional to new library: base.
Merge pull request #36 from davidmorgan/add-optional

Add Optional type and test to collection library.