Dumper enhancements

Implement:
 * unix time parse/dump, strptime/strftime `%s` equivalent
 * timezone dump casting - specification of a timezone in a custom dump
 format such as `CCYY-MM-DDThh:mm:ss+02:00` will cast the `TimePoint`
 into that timezone (this already happened for `Z`).
 * dump-as-parsed switch in the `TimePointParser.parse` method.

@matthewrmshin, please review (still running the tests...).