Adds jQuery promise support

The `init` function now returns a promise, so that it can also be used in a `$.when()` for example.

    $.when($.get('/something'), $.jsperanto.init()).done(function(){
        // do sth...
    });

Also, since the callback is not needed in this case, it allows to omit the callback as first parameter in the `init` function.

This change uses `$.isPlainObject()` which has been introduced in jQuery 1.4, so the library would no longer support jQuery 1.3.2 as it's claimed in the Readme.