data implements maps
added field author to method clear() in Data
read-only method containsValue moved from Data to DataView
forEarch and putIfAbsent tests simplyfied
merge with master
added empty lines before when, then
Data.fromMap -> Data.from + repaired some bad indentations and whitespaces
merge with master
Merge pull request #11 from cleandart/dataImplementsMap

data implements maps