Feature: TimeZoneContext and GetServerTimeZones

I implemented the TimeZoneContext Header for use in EWSClient. TimeZones can also be specified for start and end time separately (when creating or updating items). Exchange understands all 'well known' time zones. Which luckily can be retrieved from Exchange using `ExchangeWebService#get_time_zones`.

I did not implement the elements to override the logic of a time zone (Periods, TransitionsGroups and Transitions).