Open new section graph questions

I have opened a new section which will include graph questions. For this
I have provided a sample implementation for a graph data structure.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Implement DFS and BFS

With this commit I provide implementations for the common graph
traversal algorithms Depth First Search and Breadth First Search.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>