Fix optionsBeforeSubmit + optionConvertHidden for non type=submit actions.

This should address #89, and possibly #93

I was running into a problem where ngBeforeSubmit wasn't being called at all.
Dug a bit deeper and found that the `submit` event wasn't being fired on
`form` when using `form[0].submit()`.  This seems to confirm it:
http://stackoverflow.com/a/18774921/1740868

Probably what was happening, is that the options worked for those using
`<input type=submit>` and failed for people using the `<div>` or `<a>` submit
buttons.

Also updated the readme to show the correct directive for the beforeSubmit option.