FormBuilder Submit Button Default

Right now, `Form::submit()` will throw an error if a value for the button is not provided.

```html
ErrorException: Warning: htmlentities() expects parameter 1 to be string, array given in /Users/Jeffrey/Desktop/laravel/vendor/laravel/framework/src/Illuminate/Support/helpers.php line 350
```

It makes sense to me to set a sensible default of 'Submit' here.