add exports, fix typo

src/Periods.jl

@@ -3,6 +3,6 @@ module Periods
 abstract type AbstractPeriod end
 
 include("SimplePeriods.jl")
-export Period
+export Frequency, Period, Year, Semester, Quarter, Month, Week, Day
 
 end # module

src/SimplePeriods.jl

 @enum Frequency Year Semester Quarter Month Week Business Day
 
-struct Period
+struct Period <: AbstractPeriod
     ordinal::Int64
     frequency::Frequency
 end
 
-function Period(year, subperiod, frequency) <: AbstractPeriod
+function Period(year, subperiod, frequency)
     if frequency == Year
         ordinal = year - 1970
     elseif frequency == Semester