Share cache across client timezones when local timezone calcaulations are not used

[jonmmease]

Follow on to #82

The local timezone string has been added as an optional field of every Task. Currently it is always included so the finger print of a task will always include the client's timezone. A future improvement would be to only include the local timezone in tasks that actually need it. This would allow local timezone agnostic tasks to share cache entries. As it currently stands, the cache will not be shared across clients in different time zones.