Move ActiveModel into concerns directory
Rename ActiveModel to Modelable

It was confusing to have a name collision.
Convert OverloadHash into Hashable concern
Convert MethodMissing into Missable concern
Add a shared concerns context
Rename Modelable to ActiveModelable

See 733b7ce4bf59d35d3eefd5dd0d58cc9133c05e88.
Move class methods into "class << self" block
Merge branch 'concerns' into v1.0.0