Cleanup functions using DateLocale

[omus]

Functions in Base.Dates that using DateLocale typically supported either taking a DateLocale instance or a string. Typically this meant that functions would use the type-assertion Union{DateLocale, String} which made the function signatures really long and needed to be wrapped. I've made the type alias Locale to make these function signatures more readable.

Additionally, the format function was updated to support all TimeType subtypes and not just Date and DateTime.