adding support for pytz timezones

A added 3 tests for EST and all existing tests are passing.

For your consideration. There was no existing issue for anything to do with it. I'm uncertain in that case if I'm supposed to create one so I will.
Issue: https://github.com/scrapinghub/dateparser/issues/5

[example of `transitional_info` in `pytz`](https://github.com/newvem/pytz/blob/f137ff00112a9682bc4e4945067b3b88f158d010/pytz/zoneinfo/America/Vancouver.py)