Tasks Backend

I think I am done. Finished tasks_controller methods, model validations/methods, basic unit testing for tasks and mailer configurations and views (fixes #106 )

Currently, all task tests are failing, with the error `NoMethodError: undefined method `name' for :task:Symbol`.

I think the problem is related to name field of admin. That made me realize several things:
1. We didn't do the separation of `first_name`, `middle_name`, `last_name` for the `Admin` class. We probably should do that.
2. Also, I didn't see any validations for the presence of name-related fields in both `Patient` and `Admin` models. Shouldn't we have them?
3. When you are reviewing, please put more attention to tasks_controller.rb and task.rb. I wasn't really sure about how I should formulate the kinda circular relationship of assigner and assignee through the `Admin` class. 
First, always use `id` or `uid` to reference an object. In this case, `id` works for admins. Second, so what it looks like here is that you'd like to use `index` to get all tasks for a given `assignee`. I'd recommend instead being more flexible...
* If `params[:assignee_id]` filter tasks by assignee
* If `params[:assigner_id]` filter tasks by assigner
* If both, filter by both
* If neither, return all tasks (we might want to have a dashboard with everyone's tasks)