Merge pull request #4 from marcvinyals/master2

Fix Dimacs tests.
A new module of utilities
Dimacs parser and KTH parsers factored out
DAG test is not repeated for Pebbling Formulas.

other changes:

- when reading a digraph in KTH format, it is checked that it is
  source to sink sorted. In that case the DAG test always gives true.

- tree graphs in KTH format are constructed according to source to
  sink order.