* makes it possible to define the required credential fields in sms provider

* show error message if configured sms provider doesn't work, instead of throwing uncatched exception

* Adds new SMS provider ASPSMS.com

* Development SMS notifications didn't work when clicking 'send now'

Caused by the session already closed before, as it's done using a ajax request
calling a Session:start() reopens the session. Might be a bit hackish, but should be fine for development

* adds UI test for switching sms provider

* Makes API-Key default credential field to prevent breaking third party plugins

* fixes issue where save button was disabled falsely