Issue 596 automatic tasks

Improve the security of the system avoiding anonymous users to create task_runs indiscriminately. 

This pull-request addresses the issue requiring the user to submit a secured cookie, that will be validated at the server before saving the task run in the database.

The volunteers participating in the applications via the web browser and using PyBossa.JS do not have to do any special, as all the cookies are generated for them, and sent back to the server without problems.