Optimize textdomain loading

As mentioned by @deckerweb, there are some plugins that use more than one textdomain. Because of this, the new version does not longer use directory names for the textdomains of plugins. It also uses a hook, which will always be triggered, when a new mofile is loaded.

It also adds the oppertunity, to overwrite translations for specific blogs in a multisite environment. Thos overwrites will be loaded additionally to the global overwrites.