Better support for I18N : fixes #40

Hello this is my first pull request ;)

I 've made some change  for the easier translation of the widget after having myself the same issue : https://github.com/asaglimbeni/django-datetime-widget/issues/40 

Now translation can work "out of the box" in this order :
1- English is default language
2- It can be overwritten by default user setting with USE_L10N -> self.to_local()
3- Both can be overwritten by manual setting through options