type checking in initialize, ===, want, regex bug in **, fix doc
define ===, modify BaseUnit#initialize, fix doc
type checking in Unit.define, Unit.find_unit, fix doc
new file and module: UnitsMixin, UnitsNumericMixin (UnitMeasures)
new file: mixin.rb
load units_mixin.rb
change expr
fix doc
include UnitMeasures
add appendix feature