Rewritten prototype $ for show/hide to jQuery

Resolves #873 
@martinpovolny, I was just referring to lines 92 and 93 having the same exact second parameter.  The code could be DRYer in this fashion (repeatable at every call to ```set_element_visible```):
```
verb = @edit[:new][:authentication][:mode] == 'amazon'
set_element_visible("amazon_div", verb)
set_element_visible("amazon_role_div", verb)
```