fix/change 'event' reference for consistency

The [documentation for event listeners](http://garstasio.github.io/ajax-form/components/ajax-form/#ajax-form.events) has some inconsistencies/errors. Namely:

- the listener's callback event is sometimes referred to as `e` and sometimes referred to as `event` between the different examples
- the listener's callback event is referred to improperly for 'submitted' (argument setup as `e`, then immediately referred to as `event` within the same function)

To remedy these problems, I made all listener events `event`.