Add test coverage

[jrcasso]

The majority of functions need some kind of unit tests. The most important functions have been tested but we endeavor to implement 100% test coverage. The sooner, the better.

This is high priority.

[jrcasso]

As of this comment, the current test coverage percentage is an abysmal 43.8%.

[jrcasso]

I've made decent strides, but not finished quite yet:

github.com/jrcasso/gograph/gograph.go:48:               CreateGraph             100.0%
github.com/jrcasso/gograph/gograph.go:55:               CreateDirectedEdge      100.0%
github.com/jrcasso/gograph/gograph.go:62:               CreateDirectedNode      88.9%
github.com/jrcasso/gograph/gograph.go:107:              CreateDirectedNodeID    100.0%
github.com/jrcasso/gograph/gograph.go:117:              DeleteDirectedEdge      100.0%
github.com/jrcasso/gograph/gograph.go:135:              FindNode                100.0%
github.com/jrcasso/gograph/gograph.go:145:              FindDirectedNode        100.0%
github.com/jrcasso/gograph/gograph.go:155:              FindNodesByValues       100.0%
github.com/jrcasso/gograph/gograph.go:174:              TopologicalSort         95.0%
github.com/jrcasso/gograph/gograph.go:217:              CreateAdjecencyMatrix   0.0%
github.com/jrcasso/gograph/gograph.go:240:              CreateIncidenceMatrix   0.0%
github.com/jrcasso/gograph/gograph.go:266:              PrintMatrix             0.0%
github.com/jrcasso/gograph/gograph.go:273:              IsAntisymmetricMatrix   0.0%
github.com/jrcasso/gograph/gograph_test_helper.go:7:    expectEqualInts         50.0%
github.com/jrcasso/gograph/gograph_test_helper.go:13:   expectEqualStrings      50.0%
github.com/jrcasso/gograph/gograph_test_helper.go:19:   describe                100.0%
github.com/jrcasso/gograph/gograph_test_helper.go:23:   it                      100.0%
github.com/jrcasso/gograph/gograph_test_helper.go:27:   context                 100.0%
total:                                                  (statements)            66.7%