twinnable relations improved :
allow to fix a context instead of using the one on the current model (usefull when the current model isn't twinnable)
allow to use front context when it's possible
Merge pull request #114 from vrcAlbert/feature/force_and_front_context_for_twinnable

Twinnable relations improved
Fix for plural i18n

The previous fix on plural i18n was not complete, this fix makes the plural work as expected.

The translate_from_file() method return the singular form if no translation were found and if the default value is null, but for the plural() method to work we need to pass a falsy value or an array. To bypass this behaviour we need to set false as default value for translate_from_file().
Merge branch 'master/dubrovka' into master/elche