set null Interval method

tasx-16
define nullary Task constructor

tasx-16
initialize schedule to new Interval

tasx-16
rename prereqs to subTasks

tasx-16
test whether task or subtasks are scheduled

tasx-16
test whether task has subtasks

tasx-16
add subtasks

tasx-16
test if task is scheduled when subtask is scheduled

tasx-16
refactor task unit test values

tasx-16
unit test addSubTask and hasSubTasks

tasx-16
omit unnecessary default constructor

tasx-16
test interval constructors

tasx-16
use grails method to add subtasks

tasx-16
Merge branch 'develop' into feature/tasx-16-subtasks

Conflicts:
	test/unit/edu/oregonstate/mist/TaskSpec.groovy
Merge pull request #7 from osu-mist/feature/tasx-16-subtasks

Feature/tasx 16 subtasks