Added a failing random graph test for the topological sorting.
Adding a 'neighbors' helper method to DirectedGraph
Fixed the implementation of topological sorting.
Merge pull request #14 from tdanford/topo-sort-debugging

Fixing the implementation of topological sorting in DAGs.
[SCHED-12] Adding a GraphScheduler

GraphScheduler takes a Graph of tasks, and executes the tasks in topological order. Tasks which are able to be executed in parallel (because neither depends on the other) are executed using different resources.
Defined TaskGraph and GraphScheduler constructor.
Outline of a StatefulScheduler trait, along with a stub for GraphScheduler.
Adding some comments.
wip commenting out a line so that it will compile.