Added support for Translation Strings as Keys

[alwintom]

This PR will add larave's Translation Strings and Keys feature to this library and resolves #69

With this update you can define translation keys at the root element of each locale, like this:

var messages = {
            'en': {
                'Welcome': 'Welcome to the site.'
            },
            'es': {
                'Hello': 'Hola',
            }
        };
        lang = new Lang({
            messages: messages
        });
        lang.setLocale('es');
        lang.setFallback('en');

This will produce the following outputs
lang.get('Welcome') ---> Welcome to the site
lang.get('Non existing') --> Non existing
lang.get('Hello') --> 'Hola`