Add support for timetz fields

While upgrading an app to rails 4, I noticed messages like this in my development log:
```unknown OID: field_name(1266) (SELECT  ...)```

In addition, the ```time_select``` helpers on the field fail with ````undefined method `min' for "12:00:00+00":String```` because the field is interpreted as a string rather than typecast to a time.
 
The warning message is produced <a href="https://github.com/rails/rails/blob/4-0-stable/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb#L754">by this code in the postgres adapter</a>. I think its well known that postgres has both `timestamp` and `timestamptz` for date and time, but for time of day only, postgres also has both `time` and `timetz`. This patch registers `timetz` which enables my application to work as expected.

This patch is targeted to 4.0, but the same fix applies in 4.1, and I have another branch of very similar name for 4.1 in my fork. 

On master, it appears another one line fix would work for <a href="https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb#L450">this code</a>, but it is untested:

```ruby
m.alias_type 'timetz', 'time'
```

I can submit a patch for master if necessary.