Refactor WorkflowEngine::jobFinished()

Do the move of VM from busyVMs to freeVMs outside of if statement since
it is done in all cases.
Rename dagStats.getCriticalPath to getCriticalPathLength

Because it actually gets the path length. Also renamed related
variables.
Rename CriticalPath.eft to earliestFinishTimes

(more readable)
Simplify CriticalPath.getCriticalPathLength

Iterate through finish times directly instead of iterating through tasks
then getting the finish times.
Merge pull request #133 from davidshepherd7/refactoring

Minor refactoring