Exposes "submit" method on form elements

If I have a form, like in:

```
from splinter.browser import Browser
browser = Browser()
browser.visit('http://www.google.com/')
my_form = browser.find_by_tag('form').first
```

the object `my_form._element` will have the `submit` method.

As we should not use `_element` since it is related to Selenium, Splinter should exposes a `submit` method in this form-object, so we can submit forms without needing to search for the submit button (or element that actually submits the form).

It is not useful when we want to test if some button, when clicked, submits the form correctly. But it's very useful when we just want to submit the form and check what is the response (and don't want to know the name of the button/element that submits the form).