Improve the loading of a textdomain.

By moving the loading of a text domain into a method, we can expand it a bit to look in the WP_LANG_DIR first for a set of translation files (this is where developers should put their own translation files, if they don't want them wiped out during a plugin update) before falling back to looking in the plugin /languages directory.

I've also included a call to this textdomain loading method within the plugin activation method, to try and ensure that strings are translatable during activation too, and not just on subsequent page reloads.