streamline behaviour of component (trapped) forms with multiple submit buttons

this is because jquery can't know what button was pressed when the 
submit event is triggered. We circumvent it adding an hidden field before
triggering the submission

BTW: don't know why the commit seems huge. I just changed a bit the trap_form
function to handle the corner case and streamlined what disableElement, 
enableElement, disableFormElements, enableFormElements do, 
along with formInputClickSelector
Merge pull request #540 from niphlod/enhancement/js_norm_buttonname

streamline behaviour of component (trapped) forms with multiple submit buttons
Merge branch 'master' of https://github.com/web2py/web2py
Merge branch 'master' into zaap_gae