tweaks to scaffoldWidget.

@jjallaire I made the tweaks to `scaffoldWidget` that were suggested. So, a widget developer can now do

```r
devtools::create('mypkg')
htmlwidgets::scaffoldWidget('mywidget')
build(".")
mywidget()
```

It will display `Hello, World`. 

However, given that it is a html widget, I was thinking a Hello World needs to be more dynamic, stressing on the interactivity JS provides. A simple example could be to use `setTimeout` and move the message across the screen. This can be done with a few lines of code. Let me know what you think. Maybe @jcheng5 @yihui have other examples that can be used here.