Add LocaleTrait to reduce code duplication

This fix #378 

A couple of notes:
* I've kept "return $this" from setLocale, but it seems odd in a traits;
* I've copied the DocBlock from LocaleAwareProvider, but I don't know if it is the best way for the documentation.

Hope this helps :)
Would it be possible to:

- move this class to the Provider namespace
- add a space after the opening tag
- add the Geocoder license header

Thanks!