New externalSubmitButton feature

Add the "externalSubmitButton" property that allows you to connect an external button with the form, be able to submit it and take all the properties of the validation, as disabled the button, etc.

An example of the utilization:

```javascript
$('form').bootstrapValidator({
    externalSubmitButtons: $('button.externalSubmit'),
    fields: {
        ...
    }
});
```