Add tz= (time zone) parameter to get_data function calls

[pclemins]

This will allow users to specify the time zone that the results are returned in. The timestamps should be localized so the user can verify. Also, this will require some smart adjustment of the start and end Dates in the API requests to make sure we get all the data we need to go from midnight - midnight in the time zone requested.

A question... should we required start and end dates to be datetimes, or even localized date times so that users have to be explicit? We can also do some checks and apply defaults if they just want to give us dates. Thoughts?