Support Buddhist calendar year for thai locale.

In :th locale year differs from standard western calendar. Now there is already 2556 year, so offset to default year is needed. Look more at http://en.wikipedia.org/wiki/Buddhist_calendar

To make this offset additional .rb locale file is used.

```ruby
>> Date.today
=> Fri, 15 Mar 2013
>> I18n.l(Date.today)
=> "15-03-2556"
>> I18n.l(Date.today, format: :long)
=> "15 มีนาคม 2556"
>> I18n.l(Date.today, format: :year)
=> "2556"
>> I18n.l(Time.now)
=> "ศ 15 มี.ค. 2556 18:05:20 +0400"
>> I18n.l(Time.now, format: :long)
=> "15 มีนาคม 2556 18:05 น."
```