Started factoring unit tests

  The unit tests that I wrote involved the use of
  TasksViewModel and PomodoroTimer together. I have started
  separating the specs for these two classes so we can have
  proper unit tests and not integration tests. Instead of
  a direct dependency to PomodoroTimer objects, the tasks
  view model specs will start using sinon stubs and mocks
  as necessary.
When a task is marked as completed, the next uncompleted tasks will be set as active
Merge branch 'master' into gh-pages
Published version that sets the next task as active when a task has been completed