Reworked the way Goteo gets translations for the admin panel

I got a few reports that the gettext work that I did, didn't work in all machines. I figured out why and it turns out that using the gettext php module wasn't such a good idea after all. I have now reworked the i18n code to avoid using the module. This version also has a locale fallback mechanism that allows cascading translations. So for example if the locale is set to `nl_BE` (Flemish Dutch), but no actual translation exists for that language but one exists for Dutch `nl` it will fallback to that one.

I had to change some stuff in `common.php` and I added a registry for global access, which isn't per se part of the translation work but it made the code less interdependent.

I also renamed config.php to config.sample.php because I got tired of having my local configuration overwritten.  ;)

This chageset was tested in 4 machines with different OS' and it works better than the last one merged.