[SSF] Members for tasks

Added an "Interested" field that would show up in the rheader/form for Tasks, and users can 'Watch' a task(i.e, assign themselves a 'following' role). However, deployment settings can be changed to allow Users to assign themselves other roles such as "Mentor", "Volunteer" that shows up as a dropdown-list along with the "Watch" button.

Correct - you should check that the user has permission to write to that table. Even better is to perhaps use the WebServices to assign the role using the RESTful API - which will do all the checking for you (Note - I've not reviewed all this code - so this will need further investigation - see: http://eden.sahanafoundation.org/wiki/S3/S3REST/URLFormat)