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.
Remove redundant trailing slash from load_plugin_textdomain path
Merge pull request #20 from GaryJones/feature/load-textdomain

Improve the loading of a textdomain.