Add traversal conjecture Makefile
Added contig confidences to GraphWalker; Cleaned up graph_walker.c

1. Added confidence calculation to GraphWalker
   * added tests in src/tests/graph_walker_tests.c
2. Moved GraphStep into seperate .c/.h files
3. moved db_graph_prev_nodes_with_mask() to db_graph.c from graph_walker.c
4. moved dna_bases_list_to_str() to dna.c from graph_walker.c
4. assemble_contigs.c prints contig confidences
5. Added new file format proposal