Adding new grunt task `data:timezones` to create list of timezones, with an additional info: countryMap.

Adding new grunt task **data:timezones** to create list of timezones, with an additional info: countryMap.
This should resolve the ticket raised at https://github.com/moment/moment-timezone/issues/158
Use-case:
1. To group timezones based on countries, so that the timezone choices shown to end users has a two step selection menu: 
a) Select a country from list of countries (Menu 1)
b) Populate the next menu with list of timezones used in the country selected in Menu 1
2. Also, this grunt task builds the timezone list from **zone1970.tab** instead of the deprecated version **zone.tab** which the library currently uses.
Ideally, we can just replace the grunt task `data:meta` with the one added in this commit `data:timezones`.
The new task creates a timezones-list.json file which is using data frm zone1970.tab, and has additional property for each zone. So its not removing any of the properties that were being added by the `data:meta` task.