Added the Offset abstract type and made TimeZone a subtype of it. This will allow us to have generic Offsets that aren't tied to specific TimeZone rules.
Additional cleanup to generalize timezones and offsets. TimeZone is now a subtype of Offsets.
Add PeriodRange for creating Ranges from Period types.
Removed inter-period conversions. Now arithmetic on Period results in an Array{Period}. Date +- Array{Period} iterates through the array applying the Period to the Date in succession.
Merge branch 'master' of https://github.com/karbarcca/Datetime.jl