Fixed #1520

It's READY to review :)

To do:

• Adding medium tests
• Making an Ended state resumable

Summary of changes:

1. Added documentation about Ended state in docs/TASKS.md

2. Ended tasks can be removed
   See scheduler/task.go#L431)
   Before trying to remove Ended task:

   $ snaptel task remove 5db259f7-3403-421d-896a-78aa15031c80
   Error stopping task:
   Task must be stopped
   

   Notice that stopping an Ended task did not work either (the state still was Ended, so it was a standpoint).

   After removing Ended task directly works fine :

   $ snaptel task remove 71cf766a-7d95-48ed-ad74-f73853545476
   Task removed:
   ID: 71cf766a-7d95-48ed-ad74-f73853545476
   
   //verification (for this example, a task list should be empty now)
   $ snaptel task list
   No task found. Have you created a task?
   

3. An Ended state migh be resumed if the schedule is still valid
   In startTask() there is a check if the schedule is still valid at this point of time, see (scheduler/scheduler.go#L487).
   Before trying to restart Ended task:

   $ snaptel task start       fc303c34-0484-4507-b0f8-7f7c526d3983
   Error starting task:
   error 0: Subscription already exists
   

   After trying to restart Ended task:
   (an approriate message will be returned if the schedule is not valid, what happens for windowed schedule)

   $ snaptel task start 71cf766a-7d95-48ed-ad74-f73853545476
   Error starting task:
   error 0: Task is ended. Cannot be restarted.
   

   Notice: if another scheduler also puts a task into an ended state, which is being proposed in work related with a single run task, it may make sense for the ended task to be restartable.

4. Added an appropriate message that tasks in Ended state cannot be stopped
   Before: trying to stop an Ended task
   Stopping an Ended task does not work - there was no message, however, state of task still was Ended

   After: trying to stop an Ended task
   An appropriate message will be returned

   $ snaptel task stop 71cf766a-7d95-48ed-ad74-f73853545476
   Error stopping task:
   error 0: Task is ended. Only running tasks can be stopped.
   

Diagram for Ended task

This diagram presents flow for Ended task (sum up all points above):

Testing done:

• manually tested
• medium tests

@intelsdi-x/snap-maintainers