#20 Support multiple (or zero) tasks for Goal
#20 filter goals without task from execution after dependencies resolution
Fix build() function documentation
#20 Remove GoalGroup class

As Goal now holds a {Task*} instead of a Task, GoalGroup are redundant
with goals with dependencies but without any tasks.
Rename exitCode object to exitCodes
Rename test files

- goalDependencies.ceylon to dependencies.ceylon
- goalExecution.ceylon to execution.ceylon
- goalOrchestration.ceylon to orchestration.ceylon
#20 tests for goals with no tasks or with multiples tasks
#20 Remove GoalGroup from documentation

Remove GoalGroup from documentation and replace them with a
Goal with dependencies but without tasks