Add leapseconds.jl file
Add a bunch more tests
Add UTCTimezone type and make UTC an instance. Timezones are now based on instances rather than types
Restrict Period constructors to Integer. This forces the user to specify what to do with floats, etc.
Update core date alogrithms to be more robust for large negative dates. We now pass tests for the entire typemin:typemax
Formatting issues
Rework firstdayofweek and lastdayofweek that now pass all tests
Add keyword to recur to specify an exclusion function.
Add Datetime with Periods constructor.
Few updates in the date parsing functions
Remove timezone functionality