Added class for depth first search using IEnumerable and IEnumerator interface.
Added constructor for DepthFirstGraphEnumerator.
Added usings to DepthFirstGraph.
Added _startNode to DepthFirstGraphEnumerator.
Added Initialize to DepthFirstGraphEnumerator.
Implemented IEnumerable.GetEnumerator() in DepthFirstGraph.
Added Stack to DepthFirstSearchEnumerator.
Implemented other trivial methods in DepthFirstGraphEnumerator.
Implemented MoveNext for DepthFirstGraphEnumerator.
Graph now has a method to get an DepthFirstGraph Enumerable.
Removed unused using in DepthFirstGraph.
Wrote two unit tests for DepthFirstGraph.
Unit test and DepthFirstGraphTests are now fixed and work properly.