Fix cyclic test for graps with ExVert vertices.

The test_cyclic_by_dfs method previously assumed that the graph being passed
in had vertices of type int. This caused an exception to be thrown when trying
to run test_cyclic_by_dfs on graphs with ExVert vertices.

This commit resolves the issue.
add requirement of directed graph for cycle testing and topological sort.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/depth_first_visit.jl
Adding cycle tests for undirected graphs.

Cycle tests using dfs only worked for directed graphs in the past. This commit
adds this functionality. The main change is the addition of a colormap for
edges in addition to the colormap for vertices.