Add ability to rely on default configuration when loading messages.

When I met query.i18n, I was glad to have a plugin guiding me into proper localization within Javascript.
So I follow the suggested architecture, supply JSON files in the folder i18n and start coding with ```$.i18n()```

Very quickly I met a tiny incomprehension: why should I specify the path and locale when loading messages ?
I though at first that as ```$.i18n().locale``` was deduced from either the html attribute ```lang```, I might not need to pass it to the load function. Then I realize also that, having JSON resources compliant with suggested architecture, the file path could have been deduced too !

Those are the reason that push me into this patch. It'll allow people to simply load messages regarding their configuration:

```javascript
$.i18n().load(); // Note: load() is still a Promise
```

The use case is quite simple: consider a static web site, localization could be achieved through jQuery.i18n.
Then, without defining the lang attribute on html tag, browsers can load messages in their internal locales (or rely on fallback english) depending on which are statically supplied by developers. So, Javascript code may be cleaner and developers should have less work to do (that make them happy ;) ).