Graph algorithms - parsing accessible vertices

Algorithms for visiting all vertices accessible from a given start vertex.

Sample partial implementation: parse.py

Issues:

1. Analyse examples with cycles and with inaccessible vertices;
2. Why is each accessible vertex visited (no accessible vertex omited)?
3. Construct and print the tree with each vertex has, as parent, the vertex from which it was visited (the root is the starting vertex)
4. Use the tree to reconstruct the path
5. Use the same algorithm implementation with the Wolf, Goat and Cabbage problem graph plugged in to solve that problem.

Note: for printing the tree, you can use the following format:

parent
    child-1
        grandchild-1-1
        grandchild-1-2
    child-2
    child-3
        grandchild-3-1
        grandchild-3-2