Timezone generator

Random timezone generator. This will pick a random timezone from the provided data file (`data/timezones.json`):

```
var tz = grand.timezone();
// => Europe/Amsterdam
```

Optionally a continent can be provided to narrow the choices:

```
var tz = grand.timezone('America');
// => America/New_York
```

The timezones are taken from [moment-timezone](https://github.com/moment/moment-timezone) which in turn parses the timezone database (tzdb): 

  - https://github.com/moment/moment-timezone/blob/develop/moment-timezone.json
  - https://github.com/eggert/tz

The list with timezones can be updated with the `scripts/fetch_timezones.json` script.

I hope this is a usefull addition to grand, I ported it from [fabricator](https://github.com/mattijs/fabricator)