Add read_timezone option which forces timezone for datetime

## Overview
Add ```read_timezone``` option. If this option given, force the timezone to datetime ruby objects when reading from Redshift.

## Background
Redshift supports only datetime without timezone, so that ActiveRecord makes datetime ruby instance with ActiveRecord default timezone. Furthermore, ```current_timestamp``` of redshift has UTC timezone.
It locks in ```config.active_record.default_timezone``` to ```:utc```, so developers have to divide applications which uses redshift from others unless keeping ```default_timezone``` to be ```:utc```.

## Proposal
```read_timezone``` option will be set to datetime value instance. By default, if not given this option, ```default_timezone``` will be set.

How does it sound to you?