updated public Dexcom .csv export
modified UserTime class and dexcom_to_ISO8601 function to print non-UTC timestamps with proper DST-adjusted UTC offsets
added command line arguments for user timezone offset (required) and pretty printing JSON (optional)
adjusted parse_timestamp to slice out timezone offset since Python apparently can't parse native ISO 8601 format! (wants {+-}HHMM instead of {+-}HH:MM)