Better attribute accessor pattern + Japanese translation

1) Prefer `self.send(attribute)` to `read/write_attribute` method. There are a number of reasons for this:
   * Mongoid allows aliased field names (so DB field is different than rails attribute alias)
   * There are a number of plugins which operate after the read_attribute method, for example field encryption

2) Simplify i18n files (can use `en.errors.xxx` instead of `en.activerecord.errors.xxx`)

3) Add Japanese translation (including spec)