Updates to the data:meta grunt task

This is a rebase of #161.

1. Pick "zone1970.tab" as its input source if available, defaults to zone.tab. (until 'tzdata2014f', only zone.tab was available, but now it's the deprecated version)
2. Output JSON of this task now has two hashes: "countries" and "zones" to facilitate consumers to deal much better with showing timezone choices.
3. Updated contribution docs to add info about "data-meta" grunt task.