CARTO provider: allow the user to fix the time showed by Torque

[AbelVM]

As of today, Torque "reprojects" the timestamp value to the local timezone, but sometimes you need to keep original timezone information and fixed values of timestamp.

AFAIK, there's at least two gears involved in this issue:

• Importer: that translates timestamps to GMT+00 to store it, losing the timezone info in the translation
• Torque: that translates again the timestamps to local (browser) time

Use case:
Study of 24h of traffic data in a city related to the different hours of the day, but the viewers may be from all of the world. The study loses all of its sense when you view it from a place where the time zone is so different, and you see the traffic peaks at 4am and valleys at 8-9am.

Possible fixes:

• Let the user choose the time zone the data will be rendered, or "auto" to use the current behaviour
• Keep the original timezone info in the dataset or its metadata

cc @nygeog @andrewxhill

[AbelVM]

CartoDB/carto.js#534

CartoDB/carto.js#929