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.
SDK version 0.8.7.0 -> 0.8.7 (pub requires 3-digit versioning)
Merge pull request #51 from cbracken/fix-sdk-ver

SDK version 0.8.7.0 -> 0.8.7 (pub requires 3-digit versioning)
Added concat(Iterable): returns the concatenation of the input Iterables
Bump version to 0.12.1
Merge pull request #52 from cbracken/add-chain

Added concat(Iterable): returns the concatenation of the input Iterables
Merge pull request #36 from davidmorgan/add-optional

Add Optional type and test to collection library.
Bumped version to 0.12.2
Renamed base to core.
Merge pull request #54 from cbracken/base-core

Renamed 'base' to 'core'
Minor README cleanup