Add timezone functions

This adds two functions:

- `getTimezones`: returns a list of timezone objects
- `getDeviceTimezone`: attempts to detect current device's timezone (for now just returns US Pacific Time, to be replaced with something else later)

@jh-bate I need this for https://github.com/tidepool-org/jellyfish/pull/17. Mind taking a look and let me know what you think? Thanks!