Add random timezone generator

Timezones are taken from data/timezones.json. Default all timezones are
considered but a continent can be specified to limit the choices.
Add timezone data script and updated data file

The scripts/fetch_timezones.js script can be used to fetch an updated
list of timezones and write it to the data file that the timezone
generator uses.
Merge remote-tracking branch 'mattijs/timezone'

Conflicts:
	README.md
	index.js