Extended init_helper() to allow more customization

 - Renamed attribute input to submit as this is more descriptive
 - Allow to give a string which is used as display text for the Submit
   button
 - Allow to give a Submit instance wich is directly used
Merge pull request #18 from jrast/feature/customizableInput

Extended "forms.FoundationFormMixin.init_helper()" to allow more customization :

* Renamed attribute input to submit as this is more descriptive
* Allow to give a string which is used as display text for the Submit button
* Allow to give a Submit instance wich is directly used