Timezone handling #10

Timezone handling in this package needs to be revisited.

In an attempt to deal with timezones, the code in pyloop_parser.py is adding timedeltas to dates that are already correct times, creating incorrect timestamps for glucose data, insulin data, and carb data.

For an example of one place this is being done: https://github.com/tidepool-org/PyLoopKit/blob/develop/pyloopkit/pyloop_parser.py#L36. Similar code is in these functions: get_carb_data, get_insulin_data, and get_glucose_data.

This causes problems further downstream in data processing. Recommend all internal timestamp handling is kept in utc. The only code that should worry about timezones is the following:

1. Converting pump data in pump time to utc. Not needed in this code; issue reports already have pump events with utc timestamps
2. Looking up basal rates to compute relative iob for a given dose. This should use the basal schedule timezone.
3. Display. If desired, the final data can be displayed in a non-utc timezone; either the user's current timezone, or the the apparent timezone in use as inferred from the issue report.