R18n support for locale date formats

I've added R18n support to use locale date formats. This includes a _locale/ folder that can be used to define the date_format and long_date_format. In the future this could also be used for other translations.

Two new configuration parameters have became available:

  * locale (default: 'en')
  * translation_path (default: File.join(Dir.pwd, '_locale'))

Last: the default format used in tests (27 Jan 2011) is changed into 10/01/2010. It might be useful to include a _locale/en.yml by default?