Add Button Element

* move most of the Submit code into Button
* make Submit extent Button

I'm not convinced that Submit should exist anymore, it's really convenient
but you can get the same result easily with

```php
$submit = new Button('Submit');
$submit->type('submit');
```

The form builder `submit` method can do the same thing.
Add button method to FormBuilder
Remove Submit element
Merge pull request #11 from colindecarlo/add-button-element

Replace submit element with generic button element