Fix issue when sending message to Slack

Seems that AngularJS is converting everything to JSON, which doesn't            
work since Slack updated their API endpoint to expect either:

- A JSON string sent as payload via POST; 
- A JSON string sent as parameter "payload" via POST.

angular-slack currently sends it as a JSON object that doesn't have the
required stuff directly. Instead, it is in a "payload" object, which
Slack doesn't parse.