Improve onTranslationNotFound method, namespace i18n module

This release improves the onTranslationNotFound method in response to issue #59.

Improvement of i18n method

It is now possible to return a value from the method that will be used as new translation for the given key. The return value can be a string or a promise.

If a return value is specified, the respective locale will be updated in the store. Hence, subsequent requests to the same key will not trigger the onTranslationNotFound method any more.

// with string as return value. this will write the new value as translation into the store
// note: synchronous resolving of keys is not recommended as this functionality
// should be implemented in a different way
Vue.use(vuexI18n.plugin, store, {
	moduleName: 'i18n',
	onTranslationNotFound (locale, key) {
		switch(key) {
		case: '200':
			return 'Everything went fine';
			break;
		default:
			return 'There was a problem';
		}
	}}
);

// with promise as return value. this will write the new value into the store,
// after the promise is resolved
Vue.use(vuexI18n.plugin, store, {
	moduleName: 'i18n',
	onTranslationNotFound (locale, key) {

		return new Promise((resolve, reject) => {
			axios.get('/api/translations/async', {locale: locale, key:key})
			.then((result) => {
				resolve(result.data);

			}).catch() {
				reject();
			}
			
		})

	}}
);

Namespacing of i18n module

The i18n module is not properly name-spaced. All actions of the module need to be called with the module prefix. ie. store.dispatch('i18n/addLocale').